278 SYSTEMATIC ZOOLOGY 



tant characteristics of the group may best be considered under 

 the two orders. 



Order 1. Selachii 



The Selachii (Gr. creXaxos, shark) include all the sharks and 

 skates, or rays. The general shape of the body of the sharks is 

 more or less spindlelike (Fig. 287). They have the typical fins 

 already described well developed. The head bears the eyes, 

 placed laterally or slightly dorsally, the two nostrils, which are 

 ventral, and the mouth, which is a transverse slit on the ventral 



FlG. 2S7. Lamna cornubica. A shark. (From Dean's Fishes.) 



side of the head provided with upper and lower jaws, which 

 bear numerous rows of teeth. The general shape of the head 

 is sometimes curiously modified, as in the hammer-headed shark, 

 which has two thick lateral processes bearing the eyes at their 

 extremities, and as in the sawfish shark, which has the head 

 greatly prolonged anteriorly, and provided with sharp, tri- 

 angular teeth on each side like a saw. 



Back of the head and just anterior to the pectoral fins are the 

 external gill slits, never covered by an operculum ; usually there 

 are five pairs, lateral in position, but in some there are six and 

 in some seven. Between the gill slits and the eve on each side 

 of the body is an opening leading from the mouth cavity or 

 pharynx to the outside ; it is called the spiracle, and corre- 

 sponds to a non-functional gill cleft. Posterior to the pelvic 

 fins is a single opening on the median line, the cloacal opening, 

 and near it a pair of smaller openings, the abdominal pores, 

 which, like the corresponding structures in the Cyclostomata, 

 open into the abdominal cavity. The cloacal opening leads into 

 a chamber, the cloaca, into which open the rectum, the genital 



