648 MANUAL OF ZOOLOGY. 



of the skull are so enormous, that it will be best not to attempt 

 here any detailed description of the skull of the Mammalia, 

 more especially as there is as yet no universal agreement even 

 as to the nomenclature to be employed. It is sufficient to re- 

 member that the skull is composed of a series of bony segments, 

 which are often regarded as modified vertebrae. The occipi- 

 tal bone carries two condyles for articulation with the first 

 cervical vertebra. The lower jaw is composed of two halves 

 or rami, which are distinct from one 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 so- 

 called 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, the enamel being occasionally wanting. 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 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, prcemolars, and molars 

 (fig. 356). "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. 



