The polycotyledonary form of placenta is found in the Ruminantia alone. Its essential character consists in the foetal villi not being uniformly distributed, but collected into patches or cotyledons which form as it were so many small placentæ (fig. 157). The foetal villi of these patches fit into corresponding pits in thickened patches of the wall of the uterus (figs. 158 and 159). In many cases (Turner), the interlocking of the maternal and foetal structures is so close that large parts of the maternal epithelium are carried away when the foetal villi are separated from the uterus. The glands of the uterus open in the intervals between the cotyledons. The character of the cotyledons differs greatly in different types. The maternal parts are cup-shaped in the sheep, and mushroom-shaped in the cow. There are from 60-100 in the cow and sheep, but only about five or six in the Roe-deer. In the Giraffe there are, in addition to larger and smaller cotyledons, rows and clusters of short villi, so that the placenta is more or less intermediate between the polycotyledonary and diffused types (Turner). A similarly intermediate type of placenta is found in Cervus mexicanus (Turner). [FIG. 157. UTERUS OF A COW IN THE MIDDLE OF PREGNANCY LAID OPEN. (From Huxley after Colin.) _V._ vagina; _U._ uterus; _Ch._ chorion; _C^1._ uterine cotyledons; _C^2._ foetal cotyledons.] [FIG. 158. COTYLEDON OF A COW, THE FOETAL AND MATERNAL PARTS HALF SEPARATED. (From Huxley after Colin.) _u._ uterus; _Ch._ chorion; _C^1._ maternal part of cotyledon; _C^2._ foetal part.] [FIG. 159. SEMI-DIAGRAMMATIC VERTICAL SECTION THROUGH A PORTION OF A MATERNAL COTYLEDON OF A SHEEP. (From Turner.) _cr._ crypts; _e._ epithelial lining of crypts; _v._ veins and _c._ curling arteries of subepithelial connective tissue.] The groups not belonging to the Ungulata which are characterized by the possession of a diffused placenta are the Sirenia, the Cetacea, Manis, and the Lemuridæ. Sirenia. Of the Sirenia, the placentation of the Dugong is known from some observations of Harting (No. 201). It is provided with a diffuse and non-deciduate placenta; with the villi generally scattered except at the poles. The umbilical vesicle vanishes early.