that of the derivation of the fins of fishes. This resolves itself at once into two problems, the origin of the median fins, which appear in the lancelets, at t

each structure must have had some element of usefulness in all its stages. In such studies we have, as H?ckel has expressed it, "three ancestral documents, paleontology, morphology, and ontogeny"-the actual history as shown by fossil remains, the sidelight derived from comparison of structures, and the evidence of the hereditary influences shown in the development of the individual. As to the first of these ancestral

repeats phylogeny," and phylogeny, or line of descent of organisms and structures, is what we are seeking. But here the repetition is never perfect, never nearly so perfect in fact as H?ckel and his followers expected to find it. The demands of natural selection may lead to the lengthening, shortening, or dist

the latter. They arise in a dermal keel which is developed in a web fitting and accentuating the undulatory motion of the body. In th

in-rays in the embryo fish, and doubtless similarly preceded the latter in geological time. In the development of fishes the caudal fin becomes more and more the seat of propulsion. T

pproach to this condition of primitive continuity, and in the embryos of almost all fishes the same condition occurs. Dr. John A. Ryder points out the fact that there are certain unexplained exceptions to this rule. The sea-horse, pipefish, and other highly modified forms do not show this unbroken fold, and it is wanting in the embryo of the top-minnow, Gambusia affinis. Nevertheless the existence of a continuous vertical fold in the embryo is the rule, almost universal. The codf

f skin, corresponding to the vertical fold which forms the dorsal, anal, and caudal. In this view the lateral fold, at first continuous, became soon atrophied in the middle, while at either end it is highly specialized, at first into an organ of direction, th

. Sharks of unknown character must have existed long before the earliest remains accessible to us. Hence the evidence of paleontology seems conflicting and uncertain. On the whole it lends most support to the fin-fold theory. In the later Devonian, a shark, Cladoselache fyleri, is found in which the paired fins are

(Newberry), restored. Upper D

and ventral fins of Clados

g resemblance to still existing forms of the family of Heterodontid?, which originates in the Permian. The existing Heterodontid? have the usual specialized form of shark-fin, with three of the basal segments especially enlarged and placed side by side, the type seen in modern sharks. Whatever the primitive form of shark-fin, it may well be doubted whether any one of these three (Cladoselache, Pleuracanthus, or Heterodontus) ac

shark, Chiloscyllium. (

ons. The pectorals of Lepisosteus originate in the same way. Of the paired fins, the pectoral or anterior pair seems to be the first to be developed, the ventral or pelvic pair often not making its appearance until aft

body, but does not ascend into it. It begins to develop as a very low fold, hardly noticeable, and, as growth proceeds, its base does not expand antero-posteriorly, but tends rather to become narrowed, so that it has a pedunculated form. With the progress of this process the margin of the fin-fold also becomes thinner at its distal border, and at the basal part mesodermal cells make their a

preanal part of the median vertical fold which becomes anal, caudal, and dorsal. But in the top-minnow (Gambusia), of the order Haplomi, the ventral first appears as "a little papilla and not as a fold, where the body-walls join the hinder upper portion of the yol

his theory, the skeletal arrangements of the vertebrate limb are derived from modifications of one primitive form, a structure made up of successive joints, with a series of fin-rays on one or both sides of it. To this structure Gegenbaur gives the name of archipterygium. It is found in the shark, Pleuracanthus, in Cladodus, and in all the Dipnoan

ry, Mr. J. Grah

le of Neoceratodus forsteri (Gün

chipterygium is the ground-form from which all other forms of presently existing fin skeletons are derived, concerns us only in

rygium, the skeleton of the primitive paired fin, from a series of gill-rays and invol

er than the others, and, when this has happened, for the base of attachment of neighboring rays to show a tendency to migrate from the branchial arch on to the base of the larger or, as we may call it, primary ray; a condition coming about which, were

erton). Carboniferous. Family A

girdle of Acanthoe

fin of Pleuracan

ologists as plausible, is this: that "The paired limbs are persisting and exaggerated portions of a fin-fold once continuous, which stretched along each sid

ridge of thickened epiblast-of which indeed they are but exaggerations. In Balfour's own words referring to these observations: 'If the account just given of the development of the limb is an accurate record of what really ta

of Polypterus bichir. Spe

e later by Mivart-in each case based on anatomical investigation of Selachians-mainly relatin

to the primitive nature of the archipterygium, but believes that it is derived, not from the gill-septum, but from an external gill. Such a gill is well developed in the young of all

sing in part the language of Kerr, the paragraphs in quotation-marks being tak

Elasmobranch embryos, but especially in Torpedo (Narcobatus), the fin rudiments were, at an early stage, connected by a ridge of epiblast. I am not able to make out what were the other forms in which Balfour found this ridge, but subsequent research, i

tly diminishes its security as a basis on which to rest a theory. In the rays, in correlation with their peculiar mode of life, the paired fins have under

.-Arm o

successive myotomes being taken up and left behind again as the limb moved farther back. As either explanation seems an adequate one, I do not think we can lay stress upon this body of facts as supporting either one view or the other. The facts of the development of the skeleton cannot be said

inary resemblance met with between the skeletal arrangements of paired and unpaired fins. The believers in the branchial arch hypothesis have disposed of the first of these in the same way as they did the occu

tissues of the paired fins is quite secondary and does not at all agree with the metamery of the trunk. What resemblance there is may well be of a homoplastic character when we take into account the similarity in

ishes. Professor Kerr observes that this theory must be looked upon and judged: "Just as any other view at the present time regarding the nature of the vertebrate limb, rather as a speculation, brilliant and suggestive

flexure, and not at all for movements in the sagittal plane-which would be not only difficult to achieve, but would tend to alternately compress and extend its spinal cord and its viscera. Such a creature would swim through the water as does a Cyclostome, or a Lepidosiren, or any other elongated vertebrate without special swimming organs. Swimming like this, specialization for more and more rapid movem

now return to the Gegenbaur view-th

re, up to the present time received no direct support whatever of a nature comparable with that found for the rival view in the fact that, in certain forms at all events, the limbs actually do arise in the individual i

le by the great frequency with which fins apparently of this character occur amongst the oldest known fishes. On the lateral-fold view we should have to regard these as independently evolved, which would

thus decheni (Gold

en entirely removed by the brilliant work of Gegenbaur's followers, who have shown from the facts of comparative anatomy and embryology that the limbs, and the hind limbs especially, actually have undergone, and in ontogeny do undergo, an extensive backward migration. In some cases Braus has been able to find traces of this migration as far forward as a point just behind the branchial arches. Now, when we consider the numbers, the enthusiasm, and the ability of Gegenbaur's disciples, we cannot help being struck by the fact that the only evidence in

k, showing the backward migration of the gill-arches and the forward movement of

ertainty our views are regarding the morphological nature of the paired fins, and upon wha

which gradually takes its place in advance of the hindmost gill-arches. The accompanying cut is from Dean's paper, "Biometric Evidence in the Problem of the Paired Limbs of the Vertebrates" (American Naturalist for November, 1902). Dean concludes that in Heterodontus "there is no evidence that t

neither of the two views I have mentioned has a claim to any higher rank than that of extremely suggestiv

hind limbs to be beyond dispute. The great and deep-seated resemblances between them

-pouches of the gut-wall. Amongst the Urodela one such gill projects from each of the first three branchial arches. In Lepidosiren there is one on each of the branchial arches I-IV. In Polypterus and Calamoichthys (Erpetoichthys) there is one on the hyoid arch. Finally, in many Urodelan larv? we have present at the same time as the external gills a pair of curious structures called balancers. At an early stage of my work on Lepidosiren, while looking over other vertebrate embryos and larv? for purposes of comparison, my attention was arrested by these structures, and further examin

nother way. The groups in which they do not occur are those whose young possess a very large yolk-sac (or which are admittedly derived from such forms). Now wherever we have a large yolk-sac we have developed on its surface a rich network of blood-vessels for purp

extremely liable to injury and consequently a source of danger to their possessor. Their absence, therefore, in certain cases may well have been due to natural selection. On the other hand, the presence in so many lowly forms of these organs, the general close similarity in structure that runs th

ill may be the structure modified to form the paired limbs. Of the homology of

ators, depressors, and adductors, and larv? possessing them may be seen every now and then to give them a sharp backward twitch. They are thus potentially motor organs. In such

erygian fish from the Congo River. Young

l arches; it was inherently extremely probable that these should be made use of when actual supporting, and motor appendages had to be developed in connection with clamber

the other two hypotheses is that, instead of deriving the swimming-fin from the walking and supporting limb, it goes the other way about. That this is the safer line to take seems to me to be shown by the consideration that a very small and rudimentary limb could only be of use if provided with a fixed point d'appui. Als

uld be decisive. At present we have to decide which is the more primitive of two forms of pectoral fin actually known among fossils. That of Cladoselache is a low, horizontal fold of skin, with feeble rays, called by Cope ptychopterygium. That of Pleuracanthus is a jointed paddle-shaped appendage with a fringe of

. In this view there is no real homology between the archipterygium in the sharks possessing it and that in the Dipnoans and Crossopterygians. In the one theory the type of Pleuracanthus would be ancestral to the other sharks on the one hand, and to Crossopterygia

errated spine, evidence of a far from primitive structure. Certainly neither the one genus nor the other actually represents the primitive shark. But as Cladoselache appears in geological time, long before Pleuracanthus, Cladodus, or any other shark with a jointed, archipterygial fin, the burden of proof, according

of Sturgeon, Acipenser stur

above stated, older than the paired fins or limbs, whatever be the origin of the latter. They a

fin-rays, appear at intervals. In those fins of most service in the movement of

dult, by an undulatory motion, the paired fins tend to disappear,

mitive, the end of the tail is directed upward, and the most of the caudal fin is below it. Such a tail is seen in most sharks, in the sturgeon, garpike, bowfin, and in the Ganoid fishes. It is known as hetero

ercal and homocercal tail, this matter has been the subject of elaborate investigation

n the Report of the U. S. Fish Commissioner for 1884. In this paper a dynamic or mechanical theory of th

dal fin which consists of a rayless fold of skin continuous with the skin of the tail, the inner surfaces of this fold being more or less nearly in contact. To the same type of tail Dr. Jeffries Wyman in 1864 gave the name pr

e supposition that it represented the adult condition of the lancelet. In this creature, howe

" The term is used for the primitive type of tail in which the vertebr?, lying horizontally, grow progressively smaller, as in Neoceratodus, Protopterus, and other Dipnoans

l of Bowfin, Amia calva (

tail of Garpike, Lepis

rcal or leptocercal by a process of degeneration, or in ordinary fishes becoming homocercal. Dr. Ryder uses the term heterocercal for all cases in which any up-bending of the axis takes place, even though it involves the modification of but a single vertebra. With this definition, the tail

nus (Goode and Bean), showing

n lophocercal, then diphycercal, then heterocercal, and lastly homocercal. A similar order is indicated by the sequence of fossil fishes in the rocks, although some forms of di

d, is termed isocercal by Professor Cope. This form differs little from diphycercal, ex

oung Trout, Salmo fario (Linn?u

in a point. We may, perhaps, use it for all such as are attenuate, ending in a long point or whip, as in t

be inserted on the end of this axis, thus bridging over the interval between the vertical fins, as the name (γεφ?ρο?, bridge; κ?ρκο?, tail) is inte

ail of Hake, Merlucc

il of a Flounder, Para

ted by Ryder for the exserted tip of the vertebral column, which in some larv? (Lepisosteus) and in some adult fishes (Fistularia, Chim?ra) projects beyond the caudal fin. The urosome, or posterior part of the body, must be regarded as a pro

tail of Mola mola (Li

orate attempt to work out the homologies of the bones of t

as premises that will be

determinable, ceteris paribus,

proceed from a central or

principles are embodied i

Dipnoi, an order of fishes at present represented by Lepidosiren, Protopterus, and Ceratodus,

er-girdle forms the most obvious and determinable point for

r-girdle of Amia

irdle of a Sea Catf

r limb in the Dipnoi has almost by common consent been regar

herefore the element of the shoulder-girdle with which the humerus of the Dipnoi is articulated must also be regarded as the coracoid (su

ost wholly excluded from the glenoid foramen, and above the coracoid

a must be homologous as a whole with the coraco-scapular plate of the Batrachians; that is, it is infinitely more probable that they represent, as a whole or as dismemberments therefrom, the coraco-scapular element than t

f a Sea Catfish, Sel

the shoulder-girdles of the distantly separated classes may be (to use the terms introduced by Dr. Lankester) homoplastic, but they are not homogenetic. Therefore they must b

merely ligamentous connection with the humerus-bearing element. Consequently, as an element better representing the scapula exists, the element named scapula (by Owen, Günther, etc.) cannot be the homologue of the scapula of Batrachians. On the othe

robable that this is the homologue of the sternum of Batrachians, and that in the latter that element has been still mo

of a Batfish, Ogcocepha

obtained, a comparative examination of other types of fishes successive

ening cartilage which supports the pectoral and its basilar ossicles) must be homologous. But it is evident that the external elements of the so-called ca

a Threadfin, Polydactylus ap

ing element in the other Ganoids, and the latter consequently must be also coracoid. It is equally evident, after a detailed comparison, that the single coracoid element of the Ganoids represents the three elements developed in the generalized Teleosts

and pectoral fin in the Crossopterygians (Polypterus and Calamoichthys) and other fishes, and therefore not identical respectively with the 'supraclavicle' and 'clavicle' (except in part) recognized by him in oth

ication of the term 'coracoid.' The name 'coracoid,' originally applied to the process so called in the human scapula and subsequently extended to the independent element homologous with it in birds and other vertebrates, has been more especially retained (e.g., by Parker in mammals, etc.) for the region including the glenoid cavity. On the assumption that this may be preferred by some zootomists, the preceding terms have been applied. But if the name should be restricted to the proximal element, nearest the glenoid cavity, in which ossification commences, the name paraglenal given by Dugès to the cartilagi

TNO

f the Families