1006 KEPOKT OF COMMISSIONER OF FISH AND FISHERIES. [26] 



their embryos any evidence of continuous Unfolds as I have already 

 urged elsewhere,* so greatly does heredity influence and directly mar 

 the potency of remoter ancestry, so as to induce a defective repetition 

 of the phyletic history in the course of ontogenetic development. 



The posterior paired fins or ventrals of Teleosts also develop from 

 horizontal folds (Salmo), and there is no evidence to show that their 

 plan of evolution is much different from that of the pectorals ; at any 

 rate, no elaborately-contrived archipterygium or ichthyopterygium 

 hypothesis can give us the slightest aid in settling what must be deter- 

 mined by actual investigation. While there is much reason to regard 

 not only the pectoral but also the pelvic fin of Ceratodus as very archaic, 

 it is not certain that its very i)rimitiveness may not bo correlated with 

 the primitive character of the median diphycercal tin system, and there 

 is much reason to regard the Elasmobranch paired fins as more special- 

 ized, while those of the Ganoids and Teleosts are even more so, since 

 the latter rarely have a prolonged peduncle, but, on the contrary, a 

 very short one, the osseous elements of which are much more decidetlly 

 included by the soft parts of the bodj' walls. The i)ro- meso- and meta- 

 pterygial elements (fused actinophores, or the separate actinosts of 

 Teleosts) which are the undoubted partial homologues taken together 

 of what is the basipterygial jilate in Elasmobranch embryos, which af- 

 terward subdivides into the three proximal pterygial pieces, which are 

 evidently related, not to as many somites, to judge from what we know 

 of their evolution, but each consists of several coalesced elements de- 

 rived from as many somites. Xot so in the case of the actinosts of Te- 

 leosts, which evidently consist, perhaps exclusively, of derivatives of 

 far fewer somites than the basipterygium of the Elasmobranch embryo, 

 as we know from the fact that so few somites enter into the formation 

 of their i)aired fin-folds. While it is true that a secondary segmenta- 

 tion of the pterygio-coraco-sca])ular i)late occurs in Teleosts during a 

 late stage there are forms in which it retains the form of a chondrified 

 plate, which does not develop either an osseous scajjul;! or coracoid, 

 but persists, as in the larva, as a cartilaginous lamina, with which the 

 pectoral rays articulate directly {Gastrostomus 2hn(X^. Eurypliarynx). It 

 is therefore clear how deplorably hopeless tlie attempt must ever 

 remain to determine such homologies by consulting adult structures 

 alone, if it is desired to follow the metamerism of the embryo as a basis 

 for their determination. 



We may now glance backward and see what conclusions we may le- 

 gitimately draw from the foregoing discussion of the facts. It is evi- 

 dent, in the first place, that there is a metameric relation between the 

 actinophores of the fin-rays of the paired fins. In the second place, it 



* Development of tlie Spanish mackerel {Cyhium viacidatum), Bull. U. S. Fish Com- 

 mission I, l&Sl, pp. 160-161 ; Contiib. to the Embryography of Osseous Fishes, Rep. 

 U. S. Fish Commission, 1882, p. [04] or 518. 



