. A manual of zoology. Zoology. 63i CHORD ATA. PODiD^, or kangaroos, resemble the 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. 3Iac?-opus giganteus. The Phalangistid^ have very variable teeth. They resemble in habits the squirrels, Petaunis having the same parachute folds as does our flying squirrel. The Dipro- todonta contain many fossil forms in Australia and a few in South America, Some of the Australian fossils were very large, Diprotodon austialis larger than a rhinoceros. Sub Class III. Plucental
. A manual of zoology. Zoology. 63i CHORD ATA. PODiD^, or kangaroos, resemble the 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. 3Iac?-opus giganteus. The Phalangistid^ have very variable teeth. They resemble in habits the squirrels, Petaunis having the same parachute folds as does our flying squirrel. The Dipro- todonta contain many fossil forms in Australia and a few in South America, Some of the Australian fossils were very large, Diprotodon austialis larger than a rhinoceros. Sub Class III. Plucentalia (Monodelpliia). The first reason for assoeiating the mammals of the Old ^'orld and most of tliose of the Xew together as Placentalia is an embry- ological one, the presence of a jjlacenta. When serosa, amnion, and ailantois (p. 553) have developed in the embryo, the vessels of the allautois spread out beneath the serosa, and form with this the chorion, which sends small processes or villi into the now highly vascular mucous mem- brane of the uterus in order to obtain nourishment somewhat as a tree obtains food by its roots. These villi may be distributed over the greater jiart of the sur- face (fig. C58), jiroducing the chorion frondosum, or diffuse placenta, which occurs in C'etacea, perissodactyles and many artio- dactyles (swine). On the other hand the villi may be restricted to certain places, becoming very Fr-";. 6'>8.—Diagram of mammalian em- ^, mi • • ^'4.' hryo with chorion frondosum; a/i, amiii- strong there. ihlS glVeS riSe tO otic cavity: aZ, allantois; am, amnion; , i ^n t •-\ ^ us, umbihcai cord; rfi, chorion; rfi£, cotyledonary, discoidal, or zouary chorionic villi; ri(/, yolk stalls; ; , yolk i ' , "m j-t „;i sac; (extra-embryonic ccelom), placentil?. io these Correspontt between chorion and amnion;»/(, serosa. ,. c ,i ^ • t • „ portions of the uterine lining wliicli are distinguished from the rest tiy becoming extrem
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