ELASMOBRANCHS. 239 



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

 sharks, grouped under Galeus and Mustelus ; the largest of all sharks, 

 commonly called Carcharias (Charcharimis), some of which have man-eat- 

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

 sharks (Zyg^nid« 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^) 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 {Lamna) 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 /S — CYCLOSPONDYLI. Calcareous deposits of the verte- 

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

 SqualiD/E, including the common dog-fishes (Acanihias 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- 

 PHORID.E, 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. Rai^ (Batoidea). 



Vertebra 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^ 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 pectinatus 

 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.e 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 



