CHORDATA — VERTEBRATA — FISHES 343 



riorly. The teeth of the former (Lamna) have small lateral 

 denticles at the base of the crown. Here likewise are placed 

 the other existing sharks, dog-fish and skates. 



Sub-class 2, Holocephali (Chimceras, etc.) 



The Holocephali (Devonian to present) differ from the pre- 

 ceding sub-class in the large compressed head, small mouth, 

 single external gill opening (the true gill-slits are covered by a 

 fold of skin), and in the separation of anal and urino-genital 

 openings. Palatoquadrate bone immovably fused with the 

 cranium (whence the name from Greek holos, whole, + cephale, 

 head) . 



The only Paleozoic forms classed with the Holocephali are 

 the ptyctodonts ; these, however, may belong with Dinichthys in 

 the class Ostracodermi. They are known by their two pairs 

 of plate-like teeth, one pair in the upper jaw and one in the 

 lower. The remainder of the Holocephali, dating back to the 

 Triassic, have two pairs of dental plates in the upper jaw oppos- 

 ing the single pair below. An example of these latter is 

 Chimaera, widespread in the present seas, called sea-cat in the 

 United States. 



Sub-class J, Dipneusti (Lung-fish) 



Respiration is by both gills and lungs (whence the name from 

 Greek di, two, -f- pneo, to breathe ; whence, too, the common 

 name of lung-fish; also called mud-fish). In pure water they 

 breathe by means of their gills, like an ordinary fish ; but in 

 stagnant or muddy water they come to the surface and draw 

 air into their lungs. The gills are like those of other fishes ; the 

 lung is morphologically the air-bladder of the Teleostomi, but 

 differs in being covered by a network of veins supplied by a 

 special artery, hence acting like a true lung. Notochord per- 

 sistent without a trace of separate vertebrae except in tail. 

 Cloaca present. Paired fins lobate, — an archipterygium 

 (see page 345 and Fig. 146). Bones mostly cartilaginous. 



