TEETH 



241 



The nostrils in all Dipnoi are on the ventral surface of the snout. 

 There are separate anterior or external nares leading into the 

 nasal chambers, and posterior or internal nares opening from these 

 into the mouth (Fig. 207). These separate openings are formed in 

 the embryo by the arching over of the nasal grooves, and are doubt- 

 less correlated with the air-breathing function of the oesophageal 

 bladder. 



In the Devonian forms the nostrils are in the same position, 

 and the blunt snout is covered dorsally by a bony shield, probably 



m 



prf. 



Fio. 210. 



Restoration of the head of Dipterus. 



other letters as 



m, maxilla ; pm, premaxilla ; p, median plate ; the 

 in Fig. 208. 



representing the fused premaxilla and maxilla, with, perhaps, the 

 nasals as well. In modern Dipnoi these bones have disappeared. 



The palate is characteristically simple and very uniform in 

 structure. A large expanded parasphenoid is flanked on both sides 

 by a ptery go-palatine bone, bearing in front a large grinding tooth. 

 Two much smaller and simpler ' vomerine ' teeth are generally 

 situated in front (Figs. 207, 2 10A). These tooth-plates are developed 

 in the embryo from rows of separate denticles which become fused 

 together by their bony bases (representing the palatine and vomer) 

 (Fig. 21 OB). 



In the lower jaw Meckel's cartilage persists, covered by an 

 angular behind, and by a large toothed splenial internally. The 

 dentary has disappeared in the Dipneumones, is vestigial in Ceratodus, 



16 



