240 



CLASSIFICATION OF VERTEBRATES. 



inflict a severe wound. Our sting-rays belong to the genus Dasyatis. In 

 the MYLIOBATID/E the anterior ends of the pectorals are free from the head, 

 forming ear-like ' cephalic fins.' On our southern coast and extending into 

 the tropics is the genus Manta, the species of which are known as devil- 

 fish. These and allied forms are among the largest of fishes. 



FIG. 242. Saw-fish, Pris- 

 tis pectinatus, ventral view, 

 after Goode. 



FlG. 243. Common skate, Raia erinacea, 

 from Agassiz and Gould. 



ORDER IV. HOLOCEPHALI. 



Cartilaginous fishes in which no true bone is developed ; tail 

 heterocercal ; gill slits 4, externally covered by a membranous 

 fold, so that but one opening appears on the outside ; skin naked ; 

 pelvic fin of male with ' clasper ' ; skull autostylic ; upper jaw 

 formed by pterygoquadrate ; optic chiasma present ; a spiral valve 

 in the intestine ; no air-bladder. 



Three genera containing about half a dozen species repre- 

 sent this group to-day. The body is much like that of the 

 sharks ; but the four gill slits are not visible from the exterior, 



