Mammalia: Classification 729 



to a small rudiment, which may be separate from the scapula but more 

 commonly is fused to it; and by presence of the corpus callosum as 

 an important adjunct of the highly developed cerebral cortex. 



These three main divisions of Mammalia have been much over- 

 named. One set of names refers to presence or absence of the cloaca. 

 An animal with a cloaca has only one posterior external aperture — 

 hence the name Monotremata (Greek trema, a hole). In the absence of 

 a cloaca, there are two external apertures, anal and urinogenital. To 

 marsupials and placentals collectively has been applied the name 

 Ditremata, but inaccurately because it ignores the existence of the 

 shallow ectodermal cloaca. Another set of names refers to the condi- 

 tion of the oviducts. The differentiation and relations of these ducts 

 in monotremes are essentially the same as in reptiles and birds. This 

 fact, along with the curiously birdlike beak of monotremes and the 

 webbed feet of the duckbill, occasioned the name Ornithodelphia 

 (Greek delphus, uterus), formerly applied to the monotremes. In 

 marsupials the two oviducts remain completely double; hence the 

 group is called Didelphia. In placental mammals there is more or 

 less extensive longitudinal union of the two ducts to form median 

 chambers, culminating in the uterus simplex (Fig. 490). Placentals are 

 therefore Monodelphia. Still another set of names is one referring to 

 the evolutionary sequence of the groups. The names are compounded 

 from the following Greek words: therion, beast; protos, first; meta, after 

 or between; eu, good. The monotremes are Prototheria, the mar- 

 supials are Metatheria, and placentals — the really "good beasts" — 

 are Eutheria. In some schemes the term Theria appears as a sort of 

 "super-subclass" comprising Metatheria and Eutheria. (In one 

 scheme, Eutheria appears as a subclass comprising two "Divisions," 

 Didelphia and Monodelphia.) 



The unfortunate avian implication of "Ornithodelphia" compels 

 discard of the "-delphia" series of names. The terms " Marsupialia " 

 and " Placentalia " merit preference because they refer to the especially 

 distinctive anatomic characteristic of their respective groups. The 

 usual synonymy of the several names is summarized below. 



MONOTREMATA PROTOTHERIA ORNITHODELPHIA 



DITREMATA THERIA 



MARSUPIALIA METATHERIA DIDELPHIA 



PLACENTALIA EUTHERIA MONODELPHIA 



Following is an outline of the main divisions of a classification of 

 modern mammals. The many classifications which have been made 



