3/8 CLASSIFICATION OF VERTEBRATES. 



are very numerous, ranging from 48 to 68. Plagiaulax from 

 the Purbeck beds (upper Jurassic) of England ; Ctenacodon, 

 American Jurassic ; Chirox and Polymastodon from the Puerco 

 (lower eocene) of America. The Australian quaternary Thy- 

 lacoleo, usually regarded as a marsupial, may belong here. 



SUB-CLASS II. EUTHERIA. 



Mammals with anus distinct from the urogenital opening ; 

 sutures of the skull well marked ; episternum reduced ; coracoid 

 not articulating with the sternum, but reduced and fused with 

 scapula ; viviparous ; mammae with teats. 



Legion I. Didelphia. 



Eutherian mammals, with small corpus callosum, usually 

 with marsupial bones (except in Thylacinus). Vaginae partially 

 or completely double. As a rule no placenta developed. 



ORDER I. MARSUPIALIA. 



Teeth always present, only one (/ 3 ) replaced by a second 

 dentition, the number usually different in upper and lower jaws ; 

 two precavae present ; mammae abdominal in position and usually 

 enclosed in a pouch in which the very immature young are placed 

 after birth. 



The order marsupialia and the legion didelphia are coexten- 

 sive. The living species are almost exclusively confined to 

 Australia and the adjacent islands ; the only exceptions being 

 the family didelphidas, which is American. Fossils, however, 

 are found in Europe as well. Forms certainly belonging to the 

 order first occur in the eocene ; but others, possibly related, date 

 from the cretaceous. The order receives its name from the 

 pouch (marsupium) in which, in most species, the young are 

 carried after birth ; but this pouch is not invariably present, the 

 young in these cases being held in the fur covering the abdom- 

 inal region. When first born the young are very immature. 

 They are transferred by the mother to the nipples, to which 

 they adhere closely. Milk is forced into their mouths by mam- 

 mary muscles, and strangulation of the young is prevented by a 



