GENERAL CHARACTERS OF THE MAMMALIA. 281 



semble the metacarpals. In tlie Euminants 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 an- 

 other by sutures, and in no other examples than the Mono- 

 tremes 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 one 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 Eeptiles, 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 articulates with the squamosal 

 element of the skull, or, in other w^ords, 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 {Ornithorhynchtis) 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 or cement, 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 im- 

 planted into distinct sockets or alveoli, which, however, are 

 very imperfect in some of the Cetacea. 



Many Mammals have only a single set of teeth through- 

 out 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 decidu- 

 ous and permanent sets of teeth do not necessarily correspond 



