12 THE ELASMOBKANCH FISHES 



bital process and both anterior and posterior projections of the pectoral come 

 in contact with, and fuse to the sides of the head and body. The fusion of the 

 forward part thus profoundly modifies the branchial area, and the clefts take 

 up a ventral position. As growth continues, the forward extensions of right 

 and left pectorals meet in front, completing the disc of the adult body. 



Form and Position of Adult Fins 



It was observed above that a determining factor in the external shape as a 

 whole is the form and position of the fins. The pectorals are of first importance 

 in such a determination. In fact sharks and rays could be separated with cer- 

 tainty by the character of the pectoral fin alone. In general the pectorals of 

 the sharks are relatively small, while those in the rays are large. But a point 

 of greater importance is the secondary fusion which, as we have just seen, the 

 pectorals of the rays make with the head during developmental stages. Border 

 types, like Squatina and Rhinohatis, which could not be separated by relative 

 size of fins alone, could be distinguished with certainty by the presence or lack 

 of such fusion. 



The caudal fin, although of less value than the pectoral, may also be a deter- 

 mining factor in external form. The axis of the caudal in the sharks, although 

 compressed, is more or less fleshy and the lobes of the fin both dorsally and 

 ventrally are well developed. In adult and specialized rays, on the contrary, 

 the entire tail may be in a more or less complete state of atrophy. In the skate 

 (fig. 20), as an example, it is a long fleshy rod; in Myliohatis, although it may 

 reach an extreme length, it is slender and whip-like (fig. 8) ; and in Pfero- 

 platea it never develops beyond the rudimentary stage. In transitional tj^pes 

 of rays, however, especially among the Pristidae and Rhinobatidae, the axis of 

 the tail is muscular, and the lobes of the caudal fin are pronounced structures, 

 although they are less well developed than are those, say, of Squatina (fig. 18) . 



Among the other fins the dorsals, which are usually- correlated with the size 

 of the tail, are more poorly developed in the rays than in the sharks. Further- 

 more, these fins in the rays, if present, generally take up a position relatively 

 far posterior to the anal segment. Myliohatis as a type is exceptional in that 

 the single dorsal fin takes a position just anterior to the sting. In the skates the 

 dorsals are far out toward the tip of the tail. In fact, in some of the rays they 

 may not inaptly be said to have migrated practically off of the end of the tail. 

 An anal fin is always lacking in the rays; hence this would be of definitive 

 value were it not wanting in a few of the sharks such as the Spinaeidae and 

 the Rhinidae. 



External Form of Fin and Its Bearing on Function 



If the pectoral fin be of sufficient extent it may perform the function of pro- 

 pulsion. But in the sharks propulsion is brought about largely by the caudal 

 fin. Two general types of locomotion may be described for the Elasmobranchs. 

 2 Excepting in the second dorsal of Lamna, Alopias, etc. 



