1 18 PROF. MIVART ON THE FINS OF ELASMOBRANCHS. [Feb. 5, 



the body in swimming ; (2) the pectoral fins join the body at too low 

 a level to abut directly on the vertebral column ; (3) such direct 

 connexion is prevented by the existence of the body-cavity. 



I believe that the limb-girdles are lateral ingrowths from the 

 paired- fin skeleton (an idea already suggested by Mr. J. K. Thacher of 

 New Haven) ; and I believe such skeleton is the modified remnant of 

 a longitudinal series of similar rays formed primitively in a continuous 

 longitudinal lateral fold. 



Similarly, I believe that the skeleton of the azygos fins is a structure 

 also formed primitively in a continuous median fold, and that the 

 dorsal rays are not outgrowths from the vertebral column. If, how- 

 ever, neural spines are homologous with the cartilaginous rays of 

 dorsal fins, then it seems to follow that the spinous processes of higher 

 animals and of man are essentially exoskeletal parts which have 

 adhered to, and grown to be connate with the axial skeleton. 



As to the caudal fin, I note a constant difference both as to the 

 number and form of the skeletal parts of its dorsal and ventral por- 

 tions, seeming to point to some genetic difference needing investi- 

 gation. 



J then proceed to the question of the homologies of piscine and 

 digit-bearing limbs, adopting the view that the preaxial margin of 

 the pectoral fin is that turned obliquely dorsad, — a matter which 

 seems established by its innervation, apart from other proof. 



I oppose the view which adopts the Ceratodus type of limb as the 

 representative of the archipterygium (Elasmobranch ventrals certainly 

 lend no vestige of support to the theory), and then consider and criti- 

 cise the successive modifications of opinion expressed by Gegenbaur 

 and Huxley. 



I regard some Rays as having pectorals hypertrophied indeed, yet 

 most like the true archipterygium (t. e. the least-modified by coales- 

 cence) of all Elasmobranch pectorals. As to Teleosteans, not only 

 do I think with Gunther 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 as seen va.Angv.illa 

 and Blennius affords us indications of a very primitive type, whence 

 the pectorals of Teleosteans on the one hand and of Elasmobranchs 

 on the other may both have been derived. 



I cannot think that the metapterygium has developed into the limb- 

 axis of the digit-bearing limb ; I believe such axis to be rather derived 

 from the mesopterygium, or (as this is sometimes absent, as in Chi- 

 loscyllium, or imperfect as in Polypterus) from the propterygium. 



The conclusions I have arrived at as to the probable genesis of 

 Vertebrate limbs are as follows : — 



(1) Two continuous lateral longitudinal folds were developed 

 similar to dorsal and ventral median longitudinal folds. 



(2) Separate, narrow, solid supports in longitudinal series, and 

 with their long axes directed more or less outwards at right angles 

 with the long axis of the body, were developed in varying extents in 

 all these four longitudinal folds. 



(3) The longitudinal folds became interrupted variously, the 



