494 ZOOLOGY. 



from tlie upp( r Jurassic beds of Wyoming Territory. (A. J. S. (3), xxi, 

 511-513). 



The Development of the Placenta and its Classificatory Value. 



Many, or rather most naturalists for the past two decades, have at- 

 tached a primary value to the modifications under which the placenta is 

 manifested, for the classification of mammals. This question was re- 

 examined early in 1881 by Dr. F. M. Balfour. He considered that "the 

 fact that in marsupials both the yolk-sac and the allantois are concerned 

 in rendering the chorion vascular, makes it a priori i^robable that this 

 was the case in the primitive types of the placentalia; and this deduction 

 is supported by the fact that in the rodentia, insectivora, and cheiroptera 

 this peculiarity of the foetal membranes is actually found." In the prim- 

 itive placentalia it is also probable that, from the discoidal allantoic 

 region of the chorion simple foetal villi, like those of the pig, projected 

 into uterine crypts ; but it is not certain how far the umbilical region 

 of the chorion, which was no doubt vascular, may have also been villous. 

 From such a primitive type of foetal membranes divergencies in various 

 directions have given rise to the types of foetal membranes found at the 

 present day." 



Eeference must be had to Professor Balfour's article for his views as to 

 the further development of the placenta, the ways in which modifications 

 have arisen, and the significance of such modifications. It can only 

 be added in this place that his conclusions are as follows : (1) The 

 rodents, insectivores, and cheiropters exemplify the closest approach to 

 " the primitive type of placenta described above" and departed least 

 from what are called the " protoplacentalia " ; (2) the lemuroids, the 

 ungulates, and the edentates, or rather their ancestors, "must have 

 branched oft" from the primitive stock before the preceding had be- 

 come distinctly differentiated "; and (3) the primates " are to be derived 

 irom a primitive lemurian type." But as to the edentates and un- 

 gulates it is a question " how far these groups arose quite indejiend- 

 ently from the lirimitive stock, or whether they may have had a nearer 

 common ancestor." The carnivores were " certainly' an oft'-shoot from 

 the primitive placental typo which was quite independent" of the 

 lemuroids, ungulates, and edentates, but at what stage cannot be 

 determined. " No important light is thrown by the placenta on the 

 affinities" of the proboscideans, cetaceans, or sireniaus, but it was 

 thought that " the character of the placenta in the latter group favors 

 the view of their being related to the uugulata." The recorder feels 

 impelled to add that it seems scarcely probable that the cetaceans and 

 sirenians have diverged from different primitive s'tocks. (P. Z. S., 1881, 

 210-212.) 



Relations of some Marstqnals. 



From analogy in the placental mammals much weight has been at- 

 tached to some of the modifications of the dentition observable among 



