MAMMALIA. DIDELPHIA. 349 



have four premolars, and six to eight molars in each mandibular 

 ramus. 



Subclass IL DIDELPHIA OR METATHERIA. 



This subclass, like the previous one, contains only a single 

 order, viz. the Marsupialia 1 ; but the forms referable to it are 

 far more numerous than in the case of the Monotremata. 



The integument is always furry, and the teeth are always 

 differentiated into incisors, canines, premolars and molars. 

 Except in Phascolomys, the number of incisors in the upper 

 and lower jaws is never equal, and the number in the upper 

 usually exceeds that in the lower jaw. There is no such 

 regular succession and displacement of teeth as in most mam- 

 mals. Sometimes the anterior teeth are diphyoclont, and as a 

 general rule the tooth commonly regarded as the last premolar 

 is preceded by a milk tooth. The majority of the permanent 

 teeth of most Marsupials are regarded as belonging to the milk 

 series for two reasons, (1) they are developed from the more 

 superficial tissues of the jaws, (2) a second set, the permanent 

 teeth, begin to develop as outgrowths from them, but after 

 wards become aborted 2 . 



The odontoid process at an early stage becomes fused with 

 the centrum of the second cervical vertebra, and the number 

 of thoraco-lumbar vertebrae is always nineteen. The skull has 

 several characteristic features. The tympanic bone remains 

 permanently distinct, and the anterior boundary of the tym- 

 panic cavity is formed by the alisphenoid. The carotid canal 

 perforates the basisphenoid, and the lachrymal canal opens 

 either outside the orbit or at its margin. There are generally 

 large vacuities in the palate. The angle of the mandible is 

 (except in Tarsipes) more or less inflected ; and as a rule the 

 jugal furnishes part of the articular surface for the mandible. 



1 See Oldfield Thomas, Brit. Mus. Cat. of Marsupialia and Monotre- 

 mata (1888). 



2 W. Kiikenthal, Anat. Anz. vi. p. 364, 1891. C. Rose, Anat. Anz. 

 vn. p. 639. 



