212 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



In a few mammals only one dentition has been retained (monophyodont) ; among 

 these may be mentioned the monotremes, sirenians and cetacea. In the marsupial 

 Myrrecobius, where the permanent dentition is greatly reduced, and in some of the 

 insectivores and rodents, a prelacteal dentition has been observed in the embryo, 

 while Rose has described traces of a prelacteal and a post-permanent dentition in 

 man. In a number of mammals (guinea pigs, many bats, etc.) the milk dentition is 

 lost before birth. 



FIG. 214. Jaws of a six month lion, after Weber. Milk teeth white, permanent dotted 

 i, incisors; c, canines; m, molars; p, premolars. 



Only a few fishes (adult Acipenser, Coregonus, etc.) lack teeth, while 

 in most they extend to the lining bones of the mouth and in some to 

 the hyoid and branchial arches (pharyngeal bones) . Usually they are 

 conical, but they may be flattened and pavement-like or even form 

 large plates, apparently by the coalescence of numbers of primitive 

 teeth (dipnoi). In the amphibians the teeth are not so widely distrib- 

 uted in the mouth, occurring on the margins of the jaws and on the 

 palatines and vomers, rarely on the parasphenoid, while they are 

 entirely lacking in Bufo and Pipa. 



Among the reptiles the turtles and some of the pterodactyls are 

 toothless; most of the others have the teeth confined to the margin of 

 the jaws, though they occur on the palatines and pterygoids in the 

 snakes and lizards, and rarely (Sphenodon) on the vomer. While the 

 conical shape prevails, the teeth present a great variety of forms, some of 

 the theriomorphs closely simulating the mammals in their heterodont 

 dentition. The teeth may be anchylosed to the summit of the jaws 



