THE DIDELPHIA. 325 



the Cetaeea ; 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 Didelpliia that the 

 two long vaginae are bent vipon themselves, their proximal 

 ends becoming applied together and dilated, and these 

 dilated portions not unfrequently communicate. Another 

 very general peculiarity of the Didelphia is the inflection 

 of the lower margin of the angle of the mandible inwards 

 into a sti'ong horizontal 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 fiu-nishes part of the articular 

 surface for the mandible. 



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 always provided with true teeth ; and, 

 usually, these teeth are readily distingiiished into incisors, 

 canines, false molars, and true molars. The canines, how- 

 ever, are absent in some genera, either in both jaws or in 

 the mandible. There are usually four tme molar teeth, 

 and, as Professor 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 Phascolomys has an 

 equal number of incisors in each jaw, the number of the 

 upper being, usually, in excess of that of the lower jaw. 



The number of the dorso-lumbar vertebrae is almost 



