558 MARSUPIALS xx. i 



and the top of the skull therefore rather flat. The orbit and temporal 

 fossa remain fully confluent and there is no post-orbital bar. The bony 

 palate is incomplete posteriorly, there being large holes in the palatine 

 portion of it. The jugal bone always reaches back to the glenoid 

 articulation of the jaw. The lower jaw, consisting of course of a single 

 dentary, has a characteristically inturned or 'inflected' inner 'angle'. 

 Other special features of the skull, not usually found in placentals, 

 are that the foramen for the optic nerve and that for the eye-muscle 

 and trigeminal ophthalmic nerves are not separated from each other 



Fig. 332. Teeth of the upper jaw of the opossum (Didelphys) showing the 



last premolar, whose place is occupied in the young by a molariform tooth. 



(From Flower and Lyddeker.) 



and that the lachrymal bone extends outside the orbit. More interest- 

 ing than these apparently trivial and unconnected diagnostic features 

 is the fact that the auditory region is not protected by the formation of 

 a bulla of the petrous bone as it is in other mammals; instead the 

 alisphenoid bone sends a wing over the middle ear. 



The teeth are not easy to interpret (Fig. 332). The incisors are 

 more numerous than in placentals, as many as 5 on each side in the 

 upper and 3 in the lower jaw. Of the cheek teeth only one, the third 

 of the series, is replaced in modern forms, and if this is regarded as 

 the last premolar we have a dentition of 3 premolars and 4 molars, as 

 against the 4 and 3 of a typical placental. However, fossil marsupials 

 with three replacing teeth have been found (p. 566) and the signifi- 

 cance of the peculiar condition of the modern forms remains obscure; 

 it may be connected with the specialization of the mouth of the young 

 for life in the pouch. The cusps of the teeth of many marsupials (e.g. 

 opossum) show a close approach to the presumed primitive plan 

 (p. 549), but in addition to the main triangle other cusps are present, 

 especially on the outside. As in placentals the herbivorous types of 

 marsupial develop grinding surfaces on the teeth. 



