MAMMALS. 381 



Legion II. Monodelphia (Placentalia). 



Eutherian mammals with well-developed corpus callosumr 

 and small anterior commissure ; no marsupial bones ; vagina 

 single ; foetus nourished by an allantoic placenta. 



ORDER I. EDENTATA (BRUTA). 



Placental mammals with the incisors, and occasionally all 

 the teeth, lacking. Teeth when present, usually prismatic ; 

 molars without enamel. Carpals and tarsals usually in linear 

 series (taxeopodous, p. 392) ; digits armed with long, com- 

 pressed, and pointed claws. 



The Edentata includes a rather heterogeneous assortment 

 of forms, the range of variation being even greater when the 

 fossils are considered. Most of the species are not strictly 

 edentulous, since molars are usually present. These are homo- 

 dont, and except in Tatusia they are monophyodont and have 

 persistent pulps. The skin is covered with hair, horny scales, 

 or bony shields, these sometimes uniting into a more or less 

 complete armor for the body. The mammae are thoracic or 

 abdominal in position. The cerebral hemispheres are small. 

 The placenta shows great variations ; it may be deciduate or 

 not ; in shape it may be diffuse, discoidal, or of discoidal lobes,, 

 or zonary. 



The edentates are given a position here at the base of the- 

 placental mammals because of their low grade of structure- 

 In some respects, as in the simple condition of the brain, this 

 low grade is primitive ; but in other respects, as in skeleton 

 and teeth, the group is clearly degenerate, although as yet it 

 is uncertain from what group they have sprung. According to* 

 Cope they have probably descended from the group of tillo- 

 dontia of the later cretaceous and eocene. The earliest fossil 

 edentates known occur in the Santa Cruz beds of Patagonia, 

 regarded by Ameghino as eocene, but by some as oligocene ; 

 and it is interesting to note that these early forms retained 

 traces of enamel upon the teeth. 



The group, as a whole, belongs to the tropics and the south- 



