110 



ON CLASSIFICATION. 



villi are gathered into bunches, or cotyledons, which in the 

 Sheep (Fig. 45) are convex, and are received into cups of the 

 mucous membrane of the uterus ; while in the Cow. on the con- 

 trary, they are concave, and fit upon corresponding convexities 

 of the uterus (Figs. 44 and 46). 



Fig. 46. 



Fig. 46. — A foetal cotyledon, C 2 , half separated from the maternal cotyledon, C\ of a Cow. 



C'h, chorion. U, uterus (after Colin). 



No one, probably, would be inclined to object to the associa- 

 tion of the orders just mentioned into one great division of the 

 Monodeljjhia, characterised by its placental structure. But such 

 grouping leaves several important points for discussion. The 

 Elephant, as Professor Owen* has shown, has a zonary placenta, 

 and the genus Hyrax has been known since the time of Home 

 to be in like case. Hence, as the elephants are commonly sup- 

 posed to be closely allied with the Pachydermata, which possess 

 diffuse, non-deciduate placentae, and as Hyrax is now generally, 

 if not universally, admitted into the same order as the Horse, 

 which has a diffuse, non-deciduate placenta, it is argued that 

 placental characters do not indicate natural affinities. A ques- 

 tion, indeed, arises, which has not been answered by those who 

 have described the placentae of Elephas and Hyrax. Is the 

 placenta of these animals simply a zone-like arrangement of villi 



* "Description of the Foetal Membranes and Placenta of the Elephant.'" 

 Philosophical Transactions, 1857. 



