242 



DIPNOI 



but larger in the early fossils (Figs. 206-7, 225). The jaws of the 

 Dipnoi have undergone profound modification in connection with the 



development of the great compound 

 tooth-plates. Correlated with this 

 are the complete fusion of the ptery go- 

 quadrate bar in front and behind with 

 the skull, the reduction of the hyo- 

 mandibular and of the marginal teeth 

 and bones of the jaws, and the growth 

 of the splenial and pterygoid bones. 

 It can scarcely be doubted that the 

 Dipnoi have been derived in remote 

 pre -Devonian times from ancestors 

 possessing a more normal palatal 

 dentition, marginal teeth and bones, 

 a detached quadrate, and a hyoman- 

 dibular of moderate size. 



pa. 



Fig. l'IOa. 



The two halves of the simple 

 cartilaginous pectoral girdle fuse 

 in the middle line in Ceratodus 

 (Fig. 212). In this and other 

 primitive Dipnoi there are paired 

 ventral clavicles overlying the coracoid region, and paired cleithra 



Ventral view of the palate of Dipterus 

 restored, a.n, anterior nostril ; p, palato- 

 pterygoid bone; ;«<, parasphenoid ; p.n, 

 posterior nostril ; p.t, palatine tooth : r.t, 

 vomerine tooth. 



Fio. 'JIOb. 



Upper dentition of a young Ceratodus Forsteri. (After Semon, from Gegenbaur, Vergl. A nut.) 

 I'. vomerine plate ; P, palatine plate. The denticles are joined at their base by a delicate bony 

 network. 



over the scapular region, 

 .skull by a post-temporal. 



The cleithrum is connected with the 

 In the Dipneumones the shoulder-girdle 



