306 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



X.— PELVIC ARCH AND FIN. 



The pelvic arch consists of two similar pieces united in the middle 

 line. The anterior part of each piece is very thin, and is produced 

 into a point at the outer angle. The posterior edge is rounded, and 

 gives articulating surfaces for the rays of the ventral fin — eight in 

 number. Posteriorly in the middle line there is a hoi*se-shoe shaped 

 cartilage, the concavity of which is directed backwards, the two 

 limbs of which give attachment to portions of the infracarinales 

 muscles. Cristse for the attachment of muscles traverse the thin 

 portion, and the posterior border is edged with cartilage. According 

 to Davidoff * these bones are not homologous with the pelvis of the 

 Elasmobranchs, but. corresf>ond to the metapterygial basalia much 

 enlarged. The pelvis of Amiwrus corresponds very closely to the 

 description of that of Barbus Jiuviatilis given by the same author, 

 the horse-shoe shaped cartilage representing the stout posterior pro- 

 cess as in that form. 



The reduction of the radialia which characterises the Teleosts 

 when compared with Elasmobranchs and Ganoids is here carried to 

 its greatest extreme, these structures being entirely absent. The 

 fin-rays have the usual character. 



Having now described the structure of the various parts com- 

 posing the skeleton of this Siluroid, it remains to point out one or 

 two generalizations with regard to it. In the first place its relation 

 to the Cyprinoids is close, as evidenced by the modifications of the 

 anterior and the tail vertebrae, and also by the relations of the audi- 

 toiw apparatus. 



Secondly, there is evidence that the Siluroids have branched off 

 at a very early period from the primordial Teleosts. This is shown, 

 as has been already stated, by the almost complete ossification of the 

 skull, and also by the extent of the specialization of the various 

 parts. The canal for the orbital muscles has almost disappeared, 

 showing that Amiurus has passed through a stage in which it 

 possessed a complete canal, a stage in which the Cyprinoids still 

 remain. The perfectness of the arrangements for the fixation of the 



1 Davidoff — Beitrage zur vergl. Anat. d. hinteren Gliedmasse d. Fische. Morph. Jarhb. VI. 

 1880. 



