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Show 212 MR. SCLATER ON BIRDS FROM BRITISH GUIANA. [Feb. I, a few suggestions as to the affinities of the orders of Mammalia indicated by the structure of the placenta. W e clearly, of course, have to start with forms which could not be grouped with any of the existing orders, but which might be called the Protoplacentalia. They probably had the primitive type of placenta described above : the nearest living representatives of the group are the Rodentia, Insectivora, and Cheiroptera. Before, however, these three groups had become distinctly differentiated, there must have branched off from the primitive stock the ancestors of the Lemuridae, the Ungulata, and the Edentata. It is obvious on general anatomical grounds that the Monkeys and M a n are to be derived from a primitive Lemurian type; and with this conclusion the form of the placenta completely tallies. The primitive Edentata and Ungulata had no doubt a diffused placenta which was probably not very different from that of the primitive Lemurs ; but how far these groups arose quite independently from the primitive stock, or whether they may have had a nearer common ancestor, cannot be decided from the structure of the placenta. The Carnivora were certainly an offshoot from the primitive placental type which was quite independent of the three groups just mentioned; but the character of the placenta of the Carnivora does not indicate at what stage in the evolution of the placental Mammalia a primitive type of Carnivora was first differentiated. No important light is thrown by the placenta on the affinities of the Proboscidea, the Cetacea, or the Sirenia; but the character of the placenta in the latter group favours the view of their being related to the Ungulata. 2. O n some Birds collected by M r . E . F. im Thurnin British Guiana. By P. L. S C L A T E R , M.A., Ph.D., F.R.S., Secretary to the Society. [Received January 17, 1881.] Mr. Everard F. im Thurn having placed in my hands for determination a series of bird-skins collected by himself or under his supervision in British Guiana in 1878 and 1879, I have had great pleasure in putting names to these specimens. Amongst the 160 species, examples of which are in the collection, are some which require a few remarks for their better identification. These are:- 1. VlREOLANIUS LEUCOTIS (Sw.). Vireolanius leucotis, Salvin, Ibis, 1878, p. 443, pi. xi. One example, obtained in June 1879 on the Mazaruni river by H. Pauli, a collector employed by Mr. im Thurn. The acquisition of this specimen is very satisfactory, as it enables |