. A manual of zoology. Zoology. IV. VERTEBRATA: MAMMALIA, PLACENTALIA Order II. Diprotodonta (Phytophaga). 561 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 wi
. A manual of zoology. Zoology. IV. VERTEBRATA: MAMMALIA, PLACENTALIA Order II. Diprotodonta (Phytophaga). 561 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 , 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 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. Placentalia {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 (somet
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