GENERAL CHARACTERS OF THE MAMMALIA. 403 



The foot consists normally of five toes connected with the 

 tarsus by means of five metatarsal bones, which closely re- 

 semble the metacarpals. In the Ruminants there are only two 

 metatarsals, and these are anchylosed in the adult, and carry 

 two toes. In the Horse there is only one complete metatarsal 

 supporting a single toe. As a rule, the number of digits in the 

 hind-limb or foot is the same as that in the fore-limb or hand ; 

 but this is not always the case. 



The cranial bones are invariably connected with one another 

 by sutures, and in no other examples than the Monotremes are 

 these sutures obliterated in the adult The occipital 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 another in the embryo, and may or 

 may not be anchylosed together in the adult. However 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 Mam- 

 mals is composed of only a single piece, and articulates 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 



