DIGESTIVE SYSTEM. 37 



Marsupiata approximate the Rodentia in the absence of canine 

 teeth, and in having sometimes, as in Phascolomys, two incisors both 

 above and below. The carnivorous Marsupiata, as Didelphis, cor- 

 respond in the structure of their teeth with the Carnivora, whose 

 molars are always furnished with more or less pointed, and frequent- 

 ly, as in Phoca, many jagged crowns. The more purely carnivor- 

 ous the animal, and the more it feeds* upon living prey, the less nu- 

 merous are the molars, one of which, the largest, constitutes what is 

 called the carnivorous tooth. The canines here become large tusks 

 or fangs. The Cats serve as an example, in which, through the pro- 

 digious development of the canines, conspicuous intervals arise in 

 the dental series. The Walrus has also very large canines (tusks). 

 The Cheiroptera and insectivorous Ferae, as the Hedgehog and Mole, 

 have broader molars, but with very pointed serrse ; they are similar 

 also in the Lemurs, the Makis and Loris ; in the Cheiroptera the 

 superior incisors are very small, and easily fall out. Among the 

 Apes, those of the Old World have the same number of molars as 

 Man (20) ; those of the New World have 24. They never how- 

 ever stand in old animals (even in the Orang-utang and Chimpan- 

 zee) in an uninterrupted row, but there are always, on account of 

 the enormous development of the canines, conspicuous spaces in 

 front of the molar teeth. In Man alone the teeth stand in one con- 

 tinuous unbroken row, and it seldom happens, save in the Negro 

 races, that small intervals remain between the incisor and canine 

 teeth of the upper jaw. It is only in an extinct race of Pachyder- 

 mata, Anoplotherium, that all the teeth form an unbroken series, as 

 in Man. 



As concerns the microscopic structure of the teeth, their tubuli 

 and enamel, &c., we are hardly prepared at present to offer any 

 generalizations, and recourse must be had therefore to the most 

 recent works of micro graphers upon this subject. The manifold 

 external forms and arrangements of the teeth are figured in zoologi- 

 cal books. 



The form of the Lips is very various. Thus many Ruminantia, 

 like the Ox, or the Sea-cow (Manati), have a thick, moist, hairless 

 upper lip, while in the Ornithorynchus hard horny kind of lips, 

 shaped like the bill of a duck, occur. Many genera possess what 

 are called cheek-pouches, that is, purse-shaped sacs, usually internal, 

 seldom external, when they are always small, as in Crelogenys and 

 Askomys. The Apes of the Old World, with the exception of the 

 highest genera, have mostly small cheek-pouches ; d r kewise some 



