30 THE MAMMALIA [SECT. A 



4. The anus and urogenital apertures are not surrounded by 

 a common sphincter. 



5. A corpus callosum is present. 



Referring, as before, to the most recent classification, we find, in the work 

 of Gregory, the following scheme of "diagnoses," which provide additional 

 osteological data. 



Sub-class Prototheria. 



Reptilian angular bone absent (at least in propria forma). 



Oviparous : no secondary teats. Shoulder-girdle (Fig. 12) and humerus 

 much as in the mammal-like reptiles : prespinous fossa of the scapula 

 absent or rudimentary, lower part of the anterior border of the scapula 

 corresponding to the spina scapulae and acromion of higher mammals ; 

 large coracoids and procoracoida overlapping in the mid- ventral line ; 

 large T-shaped interclavicle ; glenoid facet for humerus very near the 

 sternum. Epi-pubic bones present. No corpus callosum. 



Habits : primarily semi-fossorial. 

 Sub-class Theria. 



Viviparous, with teats. Prespinous fossa present (much reduced in 

 Cetacea) ; coracoid (Fig. 10) and pro-coracoid reduced, not touching 

 sternum ; interclavicle absent or vestigial ; glenoid facet for humerus 

 widely separated from sternum. 



Habits : primarily semi -arboreal. 



I. Infra-class Metatheria. 



Epi-pubic bones. No corpus callosum. Angle of mandible inflected 

 (save in Tarsipes) ; typically with only the posterior milk molar tooth 

 replaced by a successor ; p § absent ; molars typically |. 



II. Infra-class Eutheria. 



No epi-pubic bones. Corpus callosum present. Angle of mandible 

 typically not inflected ; all the antemolar teeth typically represented 

 by both milk and permanent teeth ; p § typically present ; molars 

 typically f. 



The Eutheria can be divided into at least nine Orders dis- 

 tinguished by the various combinations of morphological characters 

 enumerated in the following scheme. 



Order 1. Eutheria, in which the teeth are absent in the adult 

 or are imperfect ; the sacral vertebrae are frequently in excess 

 of the number usual in other orders. The coracoid process is 

 usually relatively larger than in other Eutheria, and does not 

 become completely fused with the scapula. The organisation of 

 the brain is very variable. Edentata. 



