SELACIIOIDEl, 353 



Order II. PLAGIOSTOMATA. 



From five to seven gill-openings. Jaws distinct from 

 skull. 



This oi'der has been divided into two suborders, which are inti- 

 mately connected with each other, and scarcely entitled to that 

 rank : — 



I. Selachoidei. Trunk gradually passing into the tail. Gill- 

 openings lateral (p. 353). 



II. Batoidei. Gili-openings ventral (p. 434). 



First Suborder. SELACHOIDEI. 

 Body more or less cylindrical, gradually passing into 

 the tail. Gill-openings lateral. — Sharks. 



SynojJsis of the Families, Groups, and Genera. 



Fam. 1. CARCHARIID^. 



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



Group A. Teeth with a single cusp with sharp edges, smooth or ser- 

 rated, erect or oblique. Snout produced longitwJinalhj. 



A. Carchariina. 



1. A pit at the root of tlic tail. 



Spiracles none 1. Carcharias, p. .357. 



Spiracles minute, pore-like. Only the upper tuetli with denticulations. 



Distinct labial folds 2. Hew.itialeus, p. 375. 



Spiracles minute. Teeth oblique, without anv denticulatiou. 



Z.' Loxodon,^. .376. 

 Spiracles small. Teeth serrated in both jaws. Caudal iin with a double 



notch 4. Galeocerdo, p. 377. 



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



with a single notch 5. Thala^sorhintis, p. 378. 



2. iVo pit at the root of the tail. 



Small spiracles 6. Galcus, p. 379. 



Group B. Teeth with a single c^tsp, ivith sharp edges, oblique. Head 

 laterally elongate, hammer-shaped. Nostrils on the front edgt 

 of the head .'. B. Zygaenina. 



Spiracles none 7. Zi/f/fpna, p. :iSO. 



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