278 THE ANATOMY OF VERTEBRATED ANIMALS. 



There is no direct comraunication between the female gen- 

 erative Orleans and the pouch ; but the minute young are 

 transported, in the blind and imperfect state in which they 

 are born, into the interior of the rnarsupmm, and each be- 

 comes attached to a nipple, which exactly fills its mouth. To 

 this it remains attached for a considerable period, the milk 

 being forced down its throat by the contraction of the cre- 

 master muscle. The danger of suffocation is averted by the 

 elongated and conical form of the upper extremity of the 

 larynx, which is embraced by the soft palate, as in the Ce- 

 tacea; and thus respiration goes on freely, while the milk 

 passes, on each side of the laryngeal cone, into the oesophagus. 



It very commonly happens among the Didelphia that the 

 two long vaginse are bent upon themselves, their proximal 

 ends becoming applied together and dilated, and these dilated 

 portions not unfrequently communicate. Another very gen- 

 eral peculiarity of the Didelphia is the inflection of the lower 

 margin of the angle of the mandible inward into a strong hori- 

 zontal process. In the genus Tarsipes, however, this process 

 is absent. 



There are further anatomical characters which are well 

 worthy of notice, though they are not so important as the 

 foregoing. 



The integument is always furry, never spiny or scaly, nor 

 provided with dermal scutes. The pinna of the external ear 

 is well developed. In the skull the carotid arteries pierce the 

 basisphenoid to enter the cranial cavity. The tympanic cavity 

 is in front, bounded by the alisphenoid ; and, very generally, 

 the jugal furnishes part of the articular surface for the man- 

 dible. 



Many of the cranial sutures, especially in the occipital 

 region, persist throughout life; and the squamosal, the united 

 poriotic ossifications, and the tympanic bones remain distinct 

 from one another. 



The jaws are always provided with true teeth ; and, usual- 

 ly, these teeth are readily distinguished into incisors, canines, 

 false molars, and true molars. The canines, however, are ab 

 sent in some genera, either in both jaws or in the mandible. 

 There are usually four true molar teeth, and, as Prof. Flower 

 has recently discovered, only one grinder succeeds another 

 vertically. It represents the last premolar. The molars never 

 possess a complex structure. 



No didelphous mammal has three incisor teeth upon each 

 side above and below ; and none but Phascolonnjs has an 



