344 



ZOOLOGY 



were once more numerous, and are known to be 

 of immense antiquity. The body is long and 

 tapering, and the thick head, with its blunt 

 snout and large eyes, has a most grotesque ap- 

 pearance. The skull is very peculiar, and the 

 vertebral column is imperfectly developed. In 

 the mouth are bony grinding plates instead of 

 teeth, and it is through the fossilization of these 

 that we know a good deal about the former 

 abundance of the group. 



Lungfishes (b) Subclass Dipneusti, or lungfishes. The skeleton, 

 though mainly cartilaginous, shows some tend- 

 ency toward ossification. There are many 

 anatomical peculiarities, but the most remark- 

 able is that of the modification of the air blad- 

 der into a sac with numerous cellular spaces, 

 which functions as a lung. Very young indi- 

 viduals have long, featherlike external gills. 

 The body is covered with scales, which super- 

 ficially resemble those of the higher fishes, 

 though differing in the details of structure. It 

 is an extraordinary thing that the scales of 

 Sagenodus, preserved in nodules about fifteen 

 million years old at Mazon Creek, Illinois, agree 

 in almost every detail with those of Neocerato- 

 dus, living today in the rivers of Queensland. 



From Dean's "Notes on Australian Lungfish " 

 FIG. 127. The Australian lungfish, Neoceratodus forsteri. 



