IV. VERTEBRATA: MAMMALIA, PLACENTALIA 

 Order II. Diprotodonta (Phytophaga). 



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The herbivorous habits are correlated with the degeneration of canines, 

 usually lacking in the lower jaw and at least very small in the upper. There 

 are two large incisors in the lower jaw, while the middle two of the upper are 

 much larger than the one or two lateral which may be present. The fact that 

 young phalangers and kangaroos are polyprotodont makes it probable that the 

 Diprotodonts are descended from the Polyprotodontia. The PHASCOLOllYlDiE 

 are the rodents of the marsupials with one chisel- like incisor in each half of each 

 jaw. Phascalomys, wombat. The Macropodid.-e, or kangaroos, resemble 

 ungulates in their large herds on the grassy places. The fore legs being very 

 small, the animals leap with the strong hind legs and tail. Macropus. The 

 PHALANGISTID.9L have very variable teeth. They resemble in habits the 

 squirrels, Petaurus having the same parachute folds as does our flying squirrel. 

 Many fossil Diprotodonta in Australia, a few in .South America. Some Aus- 

 tralian fossils were large, Diproiodon australis larger than a rhinoceros. 



Stih Class III. Placental ia {Monodelphia). 



The first reason for associating the mammals of the Old World and 

 most of those of the New together as Placentalia is an embryological one, 

 the presence of a placenta. When serosa, amnion, and allantois (p. 490) 



Fig. 609. — Rabbit embryo with envelopes (after Van Beneden and Julin"). al, 

 allantois; am, amnion; au, eye; d, digestive tract; h, heart; k, gill clefts; m, mid brain; 

 u, protovertebra;; external black, chorion with villi from which the placenta develops. 



have developed in the embryo, the vessels of the allantois spread out 

 beneath the serosa and form with this the chorion, which sends small pro- 

 cesses or villi into the now highly vascular mucous membrane of the 

 uterus in order to obtain nourishment somewhat as a tree obtains food by 

 its roots. These villi (sometimes poorly developed, as in the pig) may be 

 distributed over the greater part of the surface (lig. 609), producing the 



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