352 VERTEBRATE ZOOLOGY 



still in the nest by means of the body heat of the mother; hence there 

 is no brood pouch. When the young hatch they are fed by a milky 

 secretion which exudes from the primitive abdominal milk-glands 

 that are buried deep in the hair. The young simply licks off the 

 drops of milk as they drip from the wet hairs. When the youngsters 

 are older the mother lies on her back (Fig. 182, G) and the ludicrous 

 little fellows climb on top of her in order to feed to better advantage. 



SUB-CLASS II. EUTHERIA. (VIVIPAROUS MAMMALS) 



Definition.^ — Mammary glands are of the sebaceous type and are 

 provided with teats; brain has a corpus callosum between the cerebral 

 hemispheres; coracoid is vestigial and does not reach the sternum; 

 there is no interclavicle; ribs are double headed; vertebrae have 

 epiphyses; ovum is small; young are born alive. 



The group includes both marsupials and the placental mammals. 

 There is a much closer resemblance between the marsupials and the 

 placentals than between the former and the monotremes; hence it 

 has seemed justifiable to group the marsupials and the placentals in 

 one sub-class. 



Division I., Didelphia (Metatheria)— -Marsupials 



Definition. — Mammals with small eggs that are usually provided 

 with a thin shell and a thin layer of albumen. The oviducts are en- 

 larged into a pair of uterine pouches which are sometimes fused for a 

 short distance. The distal parts of the oviducts remain entirely 

 separate, giving a double vagina, a character responsible for the name 

 Didelphia. The egg develops in the uterus, absorbing nutriment 

 through its membranes. In rare cases a primitive allantoic placenta 

 is present. The young is born in a very immature condition and is 

 placed by the mother in the marsupium (Fig. 185, B) or brood pouch 

 (not present in all marsupials), and is fed from the milk glands by 

 means of a long tubular teat (Fig. 185, C and D) that is thrust down 

 the throat, and to which the young is attached semi-permanently 

 by means of a special larval mouth sucker. The marsupials have 

 epipubic bones; have rudimentary corpus callosum; a shallow cloaca 

 is present in at least some species. The skull has the following pe- 

 culiarities that are useful in identifying fossil species: incompletely 

 ossified palate, jugal bone reaching as far as the glenoid cavity; 

 teeth more numerous than is typical for placentals; molars generally 



