BY WILLIAM A. HASWELL, M.A., B.SC. 79 



portion of the arch is divided in the middle line in Sharks, 

 (except Squatina and Heterodontus), by a mesial more flexible 

 region which permits of a good deal of motion of the two 

 halves upon one another ; but in the Rays the two halves 

 are quite continuous with one another, the ventral portion 

 of the arch forming "(except in Toledo), a rigid bar. Borne 

 on the lateral portions of the arch towards its ventral aspect 

 are the articular surfaces for the pectoral fins. Of these there 

 are usually three, often placed in a horizontal line. Near the 

 articular surfaces are the foramina for the transmission of the 

 brachial nerves. The skeleton of the fin proper consists, when 

 typically developed, of three basal cartilages — the propterygiwni, 

 mesopterygium and metapterygium — and of a number of rays. 

 In some Sharks (Heterodontus) the propterygium is absent, and in 

 Scymnus the mesopterygium also ; in Rays the mesopterygium is 

 always small, and some of the radial cartilages may articulate 

 with the shorter-girdle directly, while the propterygium and 

 metapterygium are greatly elongated. In Sharks the pectoral fins 

 are of moderate extent and do not ai'ticulate with the cranium : 

 in Rays, on the other hand, they are greatly expanded, extend far 

 forwards as well as backwards, frequently completely encircling 

 the head, and the propterygium is, except in the Torpedinidce, 

 connected with the olfactory region of the skull, through the 

 intermediation of an ant-orbital cartilage. 



The pelvic arch is a straight or slightly curved bar of cartilage, 

 with the outer extremities of which the pelvic fins are articulated. 

 Usually it is perforated by two nerve-foramina on each side, and 

 may develope longer or shorter processes or cornua in front of and 

 behind the articular surface. Each pelvic fin contains usually 

 only two basal cartilages, the hinder being much the more 

 important, and having the greater number of the rays articulated 

 to its outer border ; connected with its distal extremity are the 

 cartilages which form the skeleton of the clasper of the male. 



The structure of the fins of the Elasmobranchii has been minutely 

 studied in connection with the subject of the origin and nature of 

 limbs. A detailed knowledge of the anatomv of these fishes seems 



