CHAPTER X 



MKMMkHA.— Continued 

 DIVISION II. MONODELPHIA (PLACENTAL MAMMALS) 



Definition. — This is the great group of present-day mammals, 

 including about 95 per cent, of all living mammalian species. They 

 are characterized by the following features: no marsupium; no epi- 

 pubic bones; the young always nourished for a considerable time in 

 the uterus by means of a placenta; no cloaca; always a good-sized 

 corpus callosum. 



The most primitive placental mammals are now believed to be 

 more nearly representative of the ancestral mammalian prototype 

 than are the monotremes or marsupials. Certain members of the 

 order Insectivora have been selected as the most generalized of living 

 mammals. Osborn selects as his mammalian prototype the tree shrew 

 Twpaia (Fig. 186, B), while Lull selects as his, Gymnura (Fig. 186, A), 

 a large rat-like animal related to the hedgehogs. The most special- 

 ized mammals are undoubtedly the whales, if structural modification 

 be taken as the criterion; but Man outranks all other mammals in 

 brain and nervous specialization, and therefore in intelligence. 



Section A. Unguiculata (Clawed Mammals) 



OEDER I. INSECTIVORA (HEDGEHOGS, MOLES AND SHREWs) 



These are primitive, rather sro.all, furry animals, that feed almost 

 exclusively on insects. They are for the most part nocturnal and 

 terrestrial in habit, as the first mammals are believed to have been. 

 Some of them have been specialized slightly for an arboreal habit; 

 others have been rather profoundly modified for a fossorial life. In 

 bodily proportions they are as a rule quite generalized, fitting well 

 the r61e usually assigned to them of persistently primitive mammals. 



The members of the Shrew family (Fig. 186, A and B) are rather 

 rat-like in form and more or less plantigrade in attitude. There is 

 nothing especially striking or noteworthy about these animals ex- 

 cept their lack of specialized characters. It has already been pointed 



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