ELASMOBRANCHS. 239 



the pectoral, and a nictitating membrane is present. Here belong the dog- 

 sharks, grouped under Galeus and Mu steins ; the largest of all sharks, 

 commonly called Carcharias (Charcharinus}, some of which have man-eat- 

 ing reputations ; and the tiger sharks, Galeocerdo. In the hammer-head 

 sharks (ZYG/ENID.E or SPHRYNID^E) the structure is much as in the Galei- 

 dae, except that the sides of the head bearing the eyes are produced into 

 lobes, giving the whole a mallet-like appearance, In the thresher-sharks 

 (ALOPIID^E) the spiracle is lacking, the last gill cleft above the pectoral, the 

 nictitating membrane absent, and the tail about as long as the rest of 

 the body. In the LAMNID/E, including the mackerel-sharks (Latnna) and 

 the great white ' man-eater ' shark (Carcharodon}, the teeth are sharp, the 

 spiracles small or absent, and the gill slits all in front of the pectoral. 



SECTION /* CYCLOSPONDYLI. Calcareous deposits of the verte- 

 bral centra arranged in one or more concentric rings about the axis. In the 

 SQUALID^:, including the common dog-fishes (Acanthias or Squalus}, the 

 fins are normal, the spiracles present, the gill openings in front of pectoral, 

 and the dorsal fins each with a spine in front. In the SQUATINID^E the 

 pectoral fins are very large, so that the body has more the shape of a large 

 flattened disk, presenting a close appearance to the skates, except that the 

 pectoral has not grown to the head. Here, too, belongs the family PRISTIO- 

 PHORiDjE, in which the snout is prolonged into a long beak, armed with 

 teeth on either side. These saw-fishes are confined to the southern hemi- 

 sphere, but may readily be distinguished from the common forms (which 

 are rays) by the position of the gill slits. 



SUB-ORDER 3. RALE (BATOIDEA). 



Vertebrae normal, cyclospondylous ; gill slits ventral in position ; spiracles 

 present; body typically flattened and rendered disk-like by the great de- 

 velopment of the pectorals. This group is greatly specialized, and is 

 apparently derived from the cyclospondylous Euselachii. 



In the PRISTID^E there is no sharp distinction between disk and tail; 

 and the rostrum is prolonged into a saw, like that of the Pristiophoridae, 

 from which, however, these saw-fish, which belong to the northern hemi- 

 sphere, may be distinguished by the position of the gills. Pristis pectinatu s 

 occasionally occurs on our southern coasts. The TORPEDINID^E, which 

 includes Torpedo, the electric skate, have the body without scales, the disk 

 rounded, and the tail thick and fleshy. A single species, known as the 

 ' cramp-fish,' occurs occasionally on our shores south of Cape Cod. The 

 electrical organ has been described (p. 115). In the RAJID/E, which in- 

 cludes our common skates, the disk is more or less rhombic in outline, rough- 

 ened by large placoid scales, two dorsals without serrated spines. Several 

 species of the genus Raja occur on the coasts. In the TRYGONID^ belong 

 the sting-rays, which have the tail usually whip-like, never two dorsals, and 

 near the base of the tail one or two serrated spines, the 'sting,' which can 



