CHARACTERS OF MAMMALIA. 545 



or rami, which are distinct from another in the embryo, and 

 may or may not be anchylosed together in the adult. How- 

 ever this may be, in no Mammal is the ramus of the lower jaw 

 composed of several pieces, as it is in Birds and Reptiles, nor 

 does it articulate with the skull by the intervention of an os 

 quadratum. On the other hand, each ramus of the lower jaw 

 in the Mammals is composed of only a single piece, and arti- 

 culates with the squamosal element of the skull, or, in other 

 words, with the squamous portion of the temporal bone. 



Teeth are present in the great majority of Mammals ; but 

 they are only present in the embryo of the whalebone Whales, 

 and are entirely absent in the genera Echidna, Manis, and 

 Myrmecophaga. In the Duck-mole (Ornithorhynchus) the teeth 

 are horny, and the same was the case in the extinct Rhytina 

 amongst the Sirenia. In all other Mammals the teeth have 

 their ordinary structure of dentine, enamel, and crusta petrosa, 

 these elements being variously disposed in different cases* In 

 no Mammals are the teeth ever anchylosed with the jaw ; and 

 in all, the teeth are implanted into distinct sockets or alveoli, 

 which, however, are very imperfect in some of the Cetacea. 



Many Mammals have only a single set of teeth throughout 

 life, and these are termed by Owen " monophyodont." In 

 most cases, however, the first set of teeth called the " milk " 

 or " deciduous " teeth is replaced in the course of growth by 

 a second set of " permanent " teeth. The deciduous and per- 

 manent sets of teeth do not necessarily correspond to one 

 another ; but no Mammal has ever more than these two sets. 

 The Mammals with two sets of teeth are called by Owen 

 " diphyodont." 



In Man and in many other Mammals the teeth are divisible 

 into four distinct groups, which differ from one another in 

 position, appearance, and function ; and which are known 

 respectively as the incisors, canines, pr&molars, and molars 

 (fig. 235). "Those teeth which are implanted in the prse- 

 maxillary bones, and in the corresponding part of the lower 

 jaw, are called ' incisors,' whatever be their shape or size. 

 The tooth in the maxillary bone which is situated at or near 

 to the suture with the prae maxillary, is the .' canine, 5 as is also 

 that tooth in the lower jaw which, in opposing it, passes in 

 front of its crown when the mouth is closed. The other teeth 

 of the first set are the ' deciduous molars ; ' the teeth which 

 displace and succeed them vertically are the * praemolars ; ' 

 the more posterior teeth, which are not displaced by vertical 

 successors, are the * molars ' properly so called " (Owen). 

 The deciduous dentition, therefore, of a diphyodont Mammal 



2 M 



