THE ELASMOBRANCH FISHES 



EXTERNAL FORM OF ELASMOBRANCHS IN GENERAL 



A comparison of the adult in two types like Acanthias, the spiny dogfish, and 

 Urolophus, the small sting ray (figs. 5 and 9), shows the two extremes of body 

 form to which the Elasmobranch fishes have diverged. In Acanthias, which is 

 beautifully adapted for cleaving the water in the running down and capture 



Fig. 16. Carcharias gJaucus. (From Garman.) 



of prey, the head is pointed, and the rounded body tapers gracefully into 

 a powerful organ of locomotion, the caudal fin; while in ZJrolophus, which 

 spends much of its time on the bottom, the head and body are depressed, carry- 

 ing the branchial clefts to a ventral position, and the pectoral fins, extending 



Fig. 17. Keterodonius francisci. 



from the pelvic fins behind to the tip of the nose in front, have become the 

 organs of effective locomotion. 



It is by differences like these that the Elasmobranch fishes have been sepa- 

 rated into two general groups (suborders) . One of these, the Selachii, contains 



Fig. 18. Squat ma calif ornica. 



the sharks; the other, the Batoidei, includes the raj'S. While this distinction 

 between the Selachii and the Batoidei is of special service in separating forms 

 like Acanthias from those like TJrolophus, yet between the two extremes there 

 are types which link by link bridge the intervening gap. So effectively is this 



