SKLACHOIDEl. 353 



Order II. PLAGIOSTOMATA. 



From five to seven gill-openings. Jaws distinct from 

 skull. 



This order has been divided into two suborders, which are inti- 

 mately connected with eacli other, and scarcely entitled to that 

 rank' : — 



I. Selachoidei. Trunk gradually passing into the tail, (iill- 

 openings lateral (p. 353). 

 IT. Batoidei. CJill-openings ventral (p. 434). 



First Suborder. SELACHOIDEI. 

 Body more or less cylindrical, gradually passing into 

 the tail. Gill-openings lateral. — Sharks. 



Sijnoj)sis of the Families, Groups, and Genera. 



Fam. 1. CARCHARIID^. 

 Eye with a nictitating membrane. An anal fin ; two dorsals. 



Group A. Teeth with a sinale cusj) with sharp edges, smooth or ser- 

 rated, erect or oblique. Snout produced longitudinally. 



A. Carchariina. 



1. A pit at the root of tlw tail. 



Spiracles none 1. Cnrcharias, p. 357. 



Spiracles minute, pore-like. Only the upper teeth with denticulatious. 



Distinct labial folds 2. Hemigaleus, p. 375. 



Spiracles minute. Teeth oblique, without any denticulation. 



3. Loxodon, p. 376. 

 Spiracles small. Teeth sen-ated in both jaws. Caudal tin with a double 



notch 4. Galeocerdo, p. 377. 



Spiracles of moderate size. Teeth serrated in both jaws. Caudal fin 



with a single notch 5. Thalassorhinus, p. 378. 



2. No pit at the root of the tai' 



Small spiracles 0. Galeus, p. 379. 



Group B. Teeth with a single cusp, with sharp edges, oblique. Head 

 laterally elongate, hammer-shaped. Nostrils on the frniit edge 

 of the head B. Zygaenina. 



Spiracles none 7. Zytfamn, p. 380. 



VPL. VIII. 2 \ 



