28 



THE MAMMALIA 



[sect, a 



5. The cloaca is not present (Gadow restricts this statement 

 to males). 



6. The oviducts are united in a longer or shorter part of their 

 course. 



7. The mammary glands are provided with teats. 



8. The ova (except in some members of Section A of the 

 Theria) are holoblastic, and the early development of the young 

 takes place in the uterus. 



9. The epi-pubic bones and the corpus callosum are variable 

 in their occurrence. 



SUB-CLASS THERIA. SECTION A. METATHERIA OR 

 MARSUPIALIA. 



1. Theria, in which the young are born in a comparatively un- 

 developed state, and are sheltered during their later development 

 in an integumentary pouch, the marsupium. 



Lachrymal bone 



Malar bone 



Glenoid 

 fossa 



Alisphenoid, 



Tympanic bone Alisphenoid 



Fig. 13 l . Cranium of Sarcophilus, a Metatherian or Marsupial Mammal; the 

 shaded area denotes the extent of the alisphenoid bone along the cranial wall ; note 

 the extent backwards of the malar bone, and the extent forwards of the lachrymal 

 bone. Cf. Fig. 14. 



2. In the skull (Fig. 13), the tympanic cavity is partly 

 bounded by the alisphenoid ; the jugal (malar) bone furnishes 

 a contribution to the floor of the glenoid cavity. 



1 In Figs. 13 and 14, a good example of "convergence" in evolution may be 

 remarked. The convergence is perhaps most definite in respect of the dentition ; 

 the latter is adapted to a carnivorous diet in both instances. In the two figures, 

 stress is laid on the osteological details serving to differentiate the two skulls, which 

 are seen to be only superficially alike. 



