MAMMALIA: METATHERIA. 369 



in Kangaroos, just within the sinus urogenitalis, forming a rudimentary 

 cloaca, and both orifices are in all cases surrounded by a common sphincter. 

 The fossa ovalis is absent in the heart, and the musculi papillares arise in 

 the right ventricle only from the septum. The testes are suspended in a 

 scrotum in front of the penis. The glans penis is bifurcate except in 

 Kangaroos, and the crura of the corpora cavernosa are either free altogether 

 from the ischia, as in most instances, or are attached to the symphysis pubis 

 by ligamentous fibres (Macropus, Hypsiprymnus), or have the usual connec- 

 tion as in a Phascogale (Sack). The left ovary is sometimes the larger of 

 the two, and the ripe Graafian follicles project from the surface of the ovary 

 to a degree rarely observable in Eutheria. The oviducts are differentiated 

 into oviducal, uterine and vaginal sections which are typically separate 

 throughout their whole extent, and open separately into the urogenital 

 sinus which is of considerable length. But in some instances the proximal 

 portions of the vaginae fuse and develope a median caecum extending to- 

 wards the urogenital sinus. The cavity of this caecum is divided by a 

 complete septum, each half being continuous with the vagina of its own 

 side in the Wombat (Phase olomys). The septum may be lost, and in a few 

 instances (e. g. Macropus Bennetti) the caecum opens direct into the uro- 

 genital sinus. The yolk sac, or umbilical vesicle, is large, fused to a limited 

 area of the subzonal membrane, and is variously stated to be vascular 

 (Osborne) or non-vascular (Caldwell). The allantois is small and vascular, 

 but it is doubtful whether it always fuses to the subzonal membrane : if 

 so, union occurs at a late period, The subzonal membrane is attached to 

 the uterine walls either by villi or by villiform pseudopodial processes of 

 the external cells (Caldwell) which are developed only from the region 

 covered by the yolk sac. The uterine glands enlarge during pregnancy, 

 and perhaps secrete a nutritive fluid. Intra-uterine life is brief, extending 

 from 2 weeks (Opossum) to 38 days (Macropus major). The number of 

 teats which are abdominal in position vary from 4 to a larger number in 

 multiparous forms, e. g. 13 in the Virginian Opossum. In this case they 

 are grouped round a central teat and not extended in lines. The marsu- 

 pial pouch for the young enclosing the teats is rudimentary, or absent in 

 some Opossums (Didelphidae). Its aperture is generally directed forwards, 

 but backwards in Thylacinus, Perameles^ and Choeropus. In the Kangaroo 

 milk is forced down the throat of the young animal by the contraction of 

 the cremaster muscle covering the mammary gland. 



The Mammalian remains, consisting chiefly of isolated teeth and lower jaws, 

 small in size, from Triassic and Jurassic strata in England and America, are generally 

 referred to this sub-class. Cf. Owen, ' British Fossil Mammals and Birds,' 1846, and 

 Palaeontographical Society's Publications, 1871 ; Marsh, American Journal of 

 Science, xx. 1880, p. 235. Fossil post-tertiary Marsupialia, some gigantic in size, 

 occur in Australia. Cf. Owen's Memoirs in Ph. Tr., republished in two vols. 1877. 



B b 



