CHAPTER XVI 



SUBPHYLUM VERTEBRATA: CLASS U. 



BRANCHII 



ELASMO- 



The elasmobranchs are the sharks, dogfish sharks, and rays 

 or skates. They resemble the true fishes (Pisces, Chapter 

 XVII) in external form, but differ from them so widely in struc- 

 ture that they are placed in a class by themselves. 1 The 

 elasmobranchs exhibit a number of structural advances over 

 the cyclostomes; there are paired fins, a lower jaw, gill arches, 

 and placoid scales. Among the peculiarities which separate 

 the elasmobranchs from the true fishes (Pisces) are the absence 

 of membrane bones, of an air bladder, and of true scales, and 

 the presence of skeletal characteristics which are not found in 

 true fishes. Two subclasses of living elasmobranchs are recog- 

 nized: the Selachh or sharks and rays, and the Holocephali 

 or chimaeras. 



i. The Dogfish Shark — Squalus acanthtas 



The common dogfish shark (Fig. 358) is abundant in the 

 waters off the coast of New England and northern Europe. 



Fig. 358. — The dogfish shark, Squalus acanthias. (From Dean, after Goode.) 



1 See Jordan, Guide to the Study of Fishes, Vol. I. pp. 506-.su- 



422 



