THE MAMMALIA 287 



With the exception of the lowest forms, mammals have 

 no cloaca. The urogenital and rectal orifices are completely 

 separated by a partition known as the perinaeum. 



An examination of the buccal cavity reveals several char- 

 acteristic features. 



The posterior nares do not open close behind the pre- 

 maxillse as in the frog, but are carried back by the development 

 of a horizontal platform of bone formed from the maxillae 

 and palatines. This partition, the hard palate, is continued 

 behind into a horizontal curtain of soft tissue, the soft palate, 

 and thus the openings of the posterior nares is carried back 

 to the throat. 



The teeth are confined to the premaxillas and maxillae in 

 the upper jaw, and in both jaws are ranged in a single series. 



They vary very much in shape and in number in different 

 mammals, and their characters afford one of the surest guides 

 in classification. In some forms — e.g. the armadillos and the 

 dolphins — the teeth are approximately of the same shape ; such 

 a dentition is called homodont. But in most mammals the 

 dentition is heterodont — i.e. the teeth are of different shapes 

 in different parts of the jaw. Thus in the dog we find on each 

 side of the upper jaw three chisel-shaped teeth or incisors, in 

 the premaxilla; a large conical tooth, the canine, situated 

 between the premaxilla and the maxilla ; and six premolar and 

 molar teeth, with prominent cusped crowns, in the maxilla. 

 The teeth in the lower jaw correspond to those in the upper 

 jaw, but there is an additional molar at the back of the series. 

 All these teeth are implanted in bony sockets, from which 

 they can easily be withdrawn in the dry skull. It can then 

 be seen that they have fangs, which are single, and taper to 

 a point in the incisors, canines, and first premolar, but are 

 double in the second and third premolars, and triple in the 

 fourth premolar and two molars. In the dog, as in the great 

 majority of mammals, the permanent dentition just described 

 is preceded by a temporary or milk dentition, in which the 

 teeth are in pattern similar to, but in number fewer, than those 

 of the permanent dentition. As a general rule, the incisors, 

 canines, and premolars have milk predecessors ; but the 

 molars only appear in the permanent dentition. In the dog 

 there is a slight exception to this rule, for the first premolar 

 has no milk predecessor. 



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