MAMMALIA: EUTHERIA. 367 



throughout their entire length (uterus duplex), e. g. Rabbits ; or fuse distally 

 having a common aperture into the vagina, but retaining an internal septum 

 in the fused portion (uterus bipartitus), e. g. most Rodentia and many 

 Chiroptera ; or they fuse throughout the greater part of their whole length 

 having a common cavity, and leaving only the oviducal portions of the 

 tube distinct (uterus bicornis), the commonest form of all ( Ungulata, Carni- 

 vora, &c.) ; or finally the uteri are completely fused (uterus simplex) as in 

 Man and Apes. The vagina is always single, and opens, as a rule, into a 

 vestibule or urogenital sinus common to it and the urethra, except where 

 the clitoris is perforated by the latter. The yolk sac, or umbilical vesicle, 

 is large in Rodentia, Insectivora, and Chiroptera, and fuses with that part of 

 the subzonal membrane left free by the allantois ; in other Eutheria it is 

 small, and does not reach the subzonal membrane. The allantois always 

 fuses with a greater or less proportion of the subzonal membrane, and 

 renders it vascular, forming the foetal placenta. This placenta is in relation 

 with the uterine walls which become thickened and more vascular during 

 pregnancy, forming the maternal placenta. The foetal placenta is furnished 

 with vascular villi, which are either simply in contact with corresponding 

 depressions in the maternal placenta, or else fuse with that structure. If at 

 the birth of the young the maternal placenta persists, or only loses epi- 

 thelium, the placenta is said to be non-deciduate ; if vascular parts of it 

 come away also, the placenta is said to be deciduate. The non-deciduate 

 placenta is either diffuse when the villi are scattered (most Ungulata, except 

 Ruminantia ; Sirenia, Cetacea, Lemuridae, among Primates) ; or cotyle- 

 donary, when they are aggregated into patches corresponding with maternal 

 patches (true Ruminantia). The deciduate placenta is either discoidal when 

 the villi are developed over a circular area and form with the maternal 

 structures a cake-like mass (Rodentia, Insectivora, Chiroptera) ; metadis- 

 coidal when the villi, at first scattered, are restricted to a limited area, as in 

 the placenta of Man and Simiidae ; or zonary when the villi are re- 

 stricted to a partial or complete girdle surrounding the embryo (Hyrax, 

 Elephant, Carnivord). The Edentata have different types of placenta : it 

 is non-deciduate and diffuse in Manis ; deciduate and zonary in Orycteropus, 

 and Dasypus novem-cinctus, or discoidal in the remainder. In Man and 

 Simiidae the ovum sinks at an early stage into the maternal mucous mem- 

 brane, which completely surrounds it, or is reflected over it. Traces of a 

 similar reflection are observable in some zonary placentae. 



The nine orders of living Eutheria are separated from one another by sub- 

 ordinate characters. They are (i) the Edentata (Sloths, Ant-eaters, Armadillos in 

 America, Cape Ant-eater in South Africa, Pangolins (Manis] in Africa and the 

 Oriental region) ; (2) the Sirenia, Manatee from the rivers flowing into the Atlantic 

 in Africa and South America : Dugong (Halicore) from the Red Sea, East of 



