366 THE ANIMAL KINGDOM. 



depression, the vestibule which represents the urogenital canal. In some 

 female Rodentia, Insectivora and Lemurs, the female urethra perforates the 

 clitoris (infra), and is entirely separate from the genital outlet. The testes 

 may be retained within the abdomen, e. g. Elephant, or pass at the breeding 

 season into a temporary scrotum, e. g. some Rodents, or be lodged perma- 

 nently in a scrotum. The termination of the vasa deferentia are often 

 furnished with vesiculae seminales. Prostatic and Cowper's glands often 

 open into the urogenital canal which enters an erectile intromittent organ 

 or penis, composed of two corpora cavernosa, and of a corpus spongiosum 

 inclosing the canal and forming a terminal glans. The two former bodies 

 are attached to the ischia, except in the Sloths and Ant-eaters, among 

 Edentata, in the Meta- and Proto-theria, and the latter is absent in the 

 Edentata just named. The ovaries are relatively small : the oviducts have 

 wide abdominal apertures, usually fringed ( = fimbriate), and are differen- 

 tiated into an oviducal portion ( = Fallopian tube), a muscular uterine por- 

 tion, and a canal or vagina. But the characters of these parts vary in the 

 three chief divisions of Mammalia. The penis of the male is represented 

 in the female by the clitoris. The ovum is relatively small : the granulosa 

 cells are very numerous, and, during the growth of the Graafian follicle, a 

 liquid the liquor folliculi appears in the centre of the follicle, while the 

 granulosa cells occupy the periphery, and bear the ovum within a small 

 projection, or discus proligerus. Segmentation is total, but slightly ir- 

 regular. 



The class Mammalia is divisible into three sub-classes Eutheria, 

 Metatheria, and Prototheria which are the equivalents of De Blainville's 

 three groups Monodelphia, Didelphia, and Ornithodelphia. 



c Mammalia,' Flower, Encyclopaedia Brit. (ed. ix.) xv j Giebel and Leche 

 Bronn's Klass. und Ordnungen des Thierreichs vi, Abth. v. (in progress) ; Hux- 

 ley, P. Z. S. 1880. Nails, Claws, Hoofs, Boas, M. J. ix. 1883 ; Gegenbaur, M. J. 

 x. 1885. Types of molar Teeth, Cope, Journal Acad. Nat. Sc. Philadelphia, viii. 

 1874-81. Mammary glands, Klaatsch, M. J. ix. 1883. Sublingua, Gegenbaur, 

 M. J. ix. 1883. Tongue, Id. M. J. xi. 1886. Osteology of Mammalia, Flower, ed. 3 

 by Gadow, 1885. 



Mammalian Descent, W. K. Parker (Hunterian Lectures), London, 1884. 

 Mammals in relation to primaeval Times, O. Schmidt, Internat. Ser. liv. 1885. 



SUB-CLASS EUTHERIA. 



THIS sub-class contains all the orders of existing Mammalia except 

 two. Its general characters are those typical of Mammalia, from which the 

 two other sub-classes deviate in certain respects. The distinctive features 

 of the reproductive system and of development are as follows. The 

 scrotum, when present, is behind the penis. The uteri either remain distinct 



