FINS OF ELASMOBEANCHS. 479 



to a possibly multiplied, but at the same time little diiferentiated, form of skeleton. 

 That they are more primitive in this matter seems evidenced by the persistence in many 

 forms of a continuous lateral fold, reaching far back along each side of the tail, and 

 seeming to be a persistent remnant of the primitive lateral continous fold from which 

 the paired limbs were developed. 



As to Teleosteans, not only do I think, with Giinther, that " the arrangement of the 

 limb-skeleton of Ceratodus is foreshadowed in the pectoral fin of Acipenser," but I 

 think it probable that the Teleostean form of limb is in some respects more primitive, 

 not only than that of Ceratodus, but than that of even any Elasmobranchs. I con- 

 sider, of course, the Teleostean limb a specially modified one, and do not believe the 

 Elasmobranch limb to have been formed from it, any more than that it was formed 

 from that of the Elasmobranchs ; but I think that in such forms as Anguilla (Parker, 

 Shoulder-girdle, pi. ii. fig. 9) and Blennius (page 46, figs, b, c) we have indications or 

 reminiscences of a more primitive type, whence Teleosteans may have been derived on 

 the one hand, and Elasmobranchs on the other. 



To return to the question as to which part of the Elasmobrach limb may most pro- 

 bably be conceived as having developed into the limb-axis of air-breathing vertebrates, 

 I think, as before said, there are some considerations which seem rather to point to 

 the line of the propterygium as that genetic part. These considerations are : — 



(1) That it is more probable that the cheiropterygium was derived from a shark- 



like form of limb than from such a fin as the pectoral fin of the Rays, which 

 (on the theory here advocated) is more primitive and more thoroughly un- 

 fitted for supporting the body on land. 



(2) That on turning the pectoral limb of a Shark ventrad in such a way that its 



dorsal surface looks outwards, as in higher vertebrates, it is the propterygium 

 which seems to be in the line of support needed for the body, the fore limb of 

 a quadruped having necessarily to extend more or less preaxiad distally. 



(3) That this circumstance renders it very unlikely that the metapterygium should 



have remained and developed into the cheiropterygial axis ; and that conse- 

 quently, if the mesopterygium is to be deemed absent in Chilosci/Uium, it can 

 only be the propterygial element which has so persisted. 



(4) The large size which the propterygium has obtained in Chimcera, Callorhynchus, 



Cestracion, ScyJlmm, and Pristiunis ^ seems to demonstrate that this element 

 is at least susceptible of great increase, and of predominance over the meta- 

 pterygial element, and so far renders it less improbable that it was the 

 genetic element of the cheiropterygial axis. 

 As to the structure of Elasmobranch ventrals, it may be unhesitatingly affirmed that 

 they lend no vestige of support to the theory that the Ceratodus-limh was the archi- 



' See Gegenbaur'B ' Untersuchimgen,' vol. ix. figs. 3, 7, 8, and 15. 



VOL. X. — PAET X. No. 6. — February 1st, 'i 87 9. 3t 



