63J: 



CHORD AT A. 



PODID.E, 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. Macropus giganteus. The PHALANGISTID.E 

 have very variable teeth. They resemble in habits the squirrels, Petaurus 

 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 

 australis larger than a rhinoceros. 



Sub. Class III. Placentalia (Monodelpliia). 



The first reason for associating the mammals of the Old World 

 and most of those of the New together as Placentalia is an embry- 

 ological one, the presence of a placenta. When serosa, amnion, 

 and allantois (p, 553) have developed in the embryo, the vessels 

 of the allantois 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 part of the sur- 

 face (fig. 658), producing the 

 chorion frondosum, or diffuse 

 placenta, which occurs in Cetacea, 

 perissodactyles and many artio- 

 dactyles (swine). On the other 

 hand the vilH may be restricted 

 to certain places, becoming very 



FIG. 658. Diagram of mammalian em- . ., _,,, . 



biTO with chorion frondosum;afo,amni- Strong there. JLhlS glVCS rise to 

 otic cavity; aZ, allantois; am, amnion; , -i j -> -j i 



as, umbilical cord; c/i, chorion; c/,z, cotyledonary, discoidal, or zonary 



chorion ic villi ; dg, yolk stalk ; ds, yolk -, m , -i T 



sac ; r, space (extra-embryonic coelom), placentae, lo these Correspond 

 between chorion and ammon;*, serosa. porti(mg rf ^ ^^ ];ning 



which are distinguished from the rest by becoming extremely vas- 

 cular (uterine placenta). The cotyledonary placenta (fig. 659) 

 consists of many small placentar patches, the cotyledons (most 

 ruminants). In the zonary placenta the villous area takes the 

 shape of a girdle or barrel (carnivores, Sirenia), while the discoidal 

 (other mammals) is, as its name indicates, disc-like. 



Besides the placental structures the higher mammals are char- 

 acterized by the disappearance of the cloaca, the unpaired vagina, 

 and absence of marsupial bones and inflected angle of the jaw. The 



eh 



