ORAL CAVITY 



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marsupial Myrmecobius, 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-per- 



FiG. 238. — Developing teeth of Acanthias. e, ingrowth of ectoderm of lower jaw; 

 eo, enamel organ; m, Meckel's cartilage; p, pulp of /, tooth. 



manent dentition in man. In a number of mammals (guinea pigs, many bats, 

 etc.) the milk dentition is lost before birth. 



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

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



I 



Fic. 339. — Jaws of a six month lion, after Weber, Milk teeth white, permanent 

 dotted. », incisors; c, canines; m, molars; p, premolars. 



some to the hyoid and branchial arches (pharyngeal bones). Usually 

 they are conical, but they may be flattened and pavement-hke or 

 even form large plates, apparently by the coalescence of numbers of 

 primitive teeth (dipnoi). In the amphibians the teeth are not so 

 widely distributed in the mouth, occurring on the margins of the jaws 



