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 Rése has described traces of a prelacteal and a post-permanent dentition in 
man. Ina number of mammals (guinea pigs, many bats, etc.) the milk dentition is 
lost before birth. 
Fic. 214.—Jaws of a six month lion, after Weber. Milk teeth white, permanent dotted. 
4, incisors; ¢, 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 
