SELACHII. 



53 



tiated as they have subsequently become. There is also some 

 doubt as to the systematic position of the typically Cretaceous 

 teeth named Ptychodus (fig. 41), which are known to have been 

 arranged in the jaws in the manner indicated in the accom- 

 panying diagram (fig. 42). Probably, however, they belong to 



Fio. 42. 



Ptychodus decurrens ; diagram of arrangement of teeth, much reduced. U. 



Cretaceous (English Chalk). A, upper jaw ; B, lower jaw. (From Brit. 

 Mus. Catal.) 



Fio. 43. 



Cyclobatis oligodactylus ; pelvic arch and fins, nat. size. U. Cretaceous; Mt. 

 Lebanon, bp., basipterygium ; il., iliac process; p.pb., prepubic process; 

 pb., pubis. 



a Mesozoic forerunner of the Myliobatidae. which were repre- 

 sented in the early Eocene by fishes having the dentition 

 indistinguishable from that of the still-surviving genera, 

 Myliobatis, Aetobatis, and Rhinoptera. The Trygonidae seem 

 to be represented in the Cretaceous by Cyclobatis, which is 



