IV. VERTEBRATA: MAMMALIA, EDENTATA. 635 



dentition, on the other hand, has undergone a progressive, diver- 

 gent development, so that the distinctions are much more pro- 

 nounced than in the marsupials, and hence of importance in 

 differentiating the orders. 



Order I. Edentata. 



A few families, poor in species, are united under the name 

 Edentata because teeth are absent or, as is more usually the case, 

 are markedly degenerate. Persistent functional incisors are lack- 



FIG. 659. Cotyledonary placenta and embryo of cow. (From Balfour, after Colin.) 

 C 1 , cotyledons of uterine, C 2 , of foetal placenta; Ch, chorion ; U, uterus; V, 

 vagina. 



ing, canines but rarely occur (Brady pus)', molars may be present, 

 sometimes in great numbers (Priodongigas, the large armadillo, has 

 about a hundred molars), but they are poorly rooted, prismatic, 

 without enamel, and usually monophyodont. Since the aardvark 

 (Orycteropus) and Tatusia have a heterodont milk dentition in 

 embryonic life in which incisors occur, and fossil edentates 

 (Entelops) with complete dentition are known, the absence of a 

 replacement of the teeth is to be explained by degeneration, which 

 may affect other parts, and is to a certain extent the reason for the 

 low position accorded these forms. The great number of sacral 

 vertebrae is striking, being as many as thirteen in some armadillos. 

 The placenta is very variable, being diffuse, discoidal, or zonary in 

 different species. The group is essentially tropical, but one species 



