130 SUBDIVISION OF MAMMALIA INTO ORDERS. 



most remarkable have already been noticed the deficiency of 

 the corpus callosum, or band of fibres connecting the two hemi- 

 spheres of the brain ; and the termination, at least among 

 the lowest of them, of the intestinal canal and the ^urinary 

 ducts, with the uterine passage, in one common receptacle, as in 

 Birds and Reptiles ( 100). In accordance with the general 

 rule, that the less the amount of assistance given to the germ 

 of a vegetable or animal in its early development, the lower is 

 the grade which it ultimately attains, we find that in many 

 points of their conformation, as well as in their degree of intelli- 

 gence, the non-placental Mammalia are much inferior to the truly 

 viviparous portion of the group. The former are further distin- 

 guished by the presence of the marsupial bones ( 90), even 

 when there is no external pouch. 



119. The Placental Mammalia are subdivided into orders 

 chiefly by the structure of their extremities, and of their teeth ; 

 with which, as has been already shown, a great part of their 

 internal conformation is closely connected ( 58). Thus we 

 distinguish the Unguiculated Mammals, which have separate 

 fingers, terminated by distinct nails or claws ; and the Ungu- 

 Iated 9 in which the fingers are more or less consolidated, and 

 inclosed at their extremity in a hard hoof. Ungulated animals 

 are necessarily herbivorous; inasmuch as the conformation of 

 their feet precludes the possibility of their seizing a living prey ; 

 and they have flat-crowned grinding teeth for triturating their 

 food. The summits of these teeth are usually not covered by a 

 smooth coat of enamel, but present a series of elevations and de- 

 pressions ; these are occasioned by the peculiar structure of the 

 teeth, which consist of alternating plates of enamel, ivory or 

 dentine, and cementum or bony substance ; and as these three 

 substances are of different degrees of hardness, their unequal 

 wear will keep the surfaces constantly rough. Among the Un- 

 guiculated Mammals, we find more variety as to the character 

 of the food. In some it is almost exclusively vegetable, as in 

 the Rodentia (Rat and Squirrel tribe) ; but here the power of the 

 separate fingers is the least possible, the bones of the fore arm 

 and leg being so constructed, that the hand cannot be turned 



