278 THE ANATOMY OF VERTEBRATED ANIMALS. 



There is no direct communication between the female gen- 

 erative organs 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 tnarsupiuin^ 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 suflbcation 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 vaginae 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 D'ldelpliia 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 

 fc-regoing. 



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

 provided wdth 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 

 periotic ossifications, and the tympanic bones remain distinct 

 from one another. 



The jaws are alwaj^s 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. Flow^er 

 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 

 ude above and below; and none but Phascolomys has an 



