28 



There are two orders in this sub-class, the Plagiostomi and the Holo- 

 >*ephali, which may be briefly distinguished as follows : 



Order PLAGIOSTOMI Dumeril. 



There are almost always five external branchial apertures, the only ex- 

 ceptions occurring in the family of Notidanoids. In Hexanchus of 

 Rafinesque, there are six. and in the Heptranchias of the same author, 

 there are seven. 



The Plagiostoms may be again divided into four suborders, the Squall, 

 the Rhince, the Prist.es and the Razee. 



Suborder SQUALI Miiller and Henle. 



The Squali are distinguished by the lateral position of the branchial 

 apertures, free supplementary eyelids, an incomplete scapular arch, and 

 the absence of a naso-pectoral cartilage. The form is always more or less 

 elongated and subcylindrical. 



In the suborder, as now restricted, only the sharks are included. The 

 Rhince or Squatince, embraced in the group by Miiller and Henle, may be 

 considered as representing another suborder. 



Suborder RHINiE Gill. 



This suborder is principally distinguished from the Squali by the de- 

 pressed head and body, and the dorsal position of the eyes. As in the 

 Squali, the branchial apertures are situated on the sides, but are placed 

 in a furrow, which separates the large and anteriorly expanded pectora: 

 fins from the body. The mouth is at the extremity of the snout. 



This group contains only one genus represented by three species. 



Suborder PRISTES Gill. 



The Pristes have the elongated and subcylindrical bodies of the Squali, 

 but the branchial apertures are open, as in the Rays, on the ventral sur- 

 face of the breast. The snout is prolonged into a long, narrow and de- 

 pressed dagger-like beak, which are provided with strong osseous spines 

 or teeth on each side. The teeth of the jaws are flat and paved. 



This suborder embraces living representatives of only one genus, which 

 must be referred to a peculiar family, that of the Pristoids. It is repre- 

 sented by a single species on the American coast. 



Suborder RAIiE Miiller and Henle. 



The RaijE have the branchial apertures beneath the body under the 

 pectoral fins and before the ventrals. The eyes and spout-holes are 

 always on the dorsal aspect of the head; the scapular arch complete, 

 and naso-pectoral cartilages present. There is no eyelid, or only an ad- 

 nate upper one. 



In this suborder, the Rays and allied fishes, with similar depressed 

 bodies, are alone embraced. 



