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; ; d.s, yolk i ' , "m j-t „;i 



sac; r.fspace (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 extremely vas- 

 cular (uterine placenta). The cotylodonary 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, Sireuia), 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 



