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Show 1 I 1 DR. J. E. GRAY ON THE BRADYPODIDA. [May 2, These three species with a spotted back and frontal band of white hair are very much alike externally, though they have differences which are not easily expressed in words; but perhaps they would be more easily defined if we had a larger series of both sexes with an accurate account of the locality which each form inhabits. They are easily distinguished by the form of the lower jaw, a character that I pointed out in my former paper published in the ' Proceedings ' of 1849. That this character is permanent and not unimportant in the economy of the animal is proved by the examination of several specimens. Thus we have four skulls of A. blainvillei, and two of A. marmoratus. The outline of the hinder part of the lower jaw of A. boliviensis is intermediate between those of A. blainvillei and A. marmoratus; but this must almost always occur when three jaws are compared together in a proper series for the purpose. This does not, however, form any ground for believing them to be variations of the same form or species. The difference of form appears to be constant when several skulls of the same species are observed. It is so, certainly, in five skulls of A. blainvillei in the British Museum. I have never seen any lower jaws that seem to me to pass by intermediate gradations from one of these forms to another. The long-produced hinder angle appears to be always in connexion with the elongate slender jaw, and the shortly produced one with the short, high, strong part of the jaw. In fact the species of this genus are very imperfectly understood, and, I believe, will prove to be more numerous than has hitherto been believed. This paper is the result of examination and re-examination of the large series of specimens and skulls and other bones of these animals in the British Museum, which has occupied me three or four hours a day for upwards of three weeks, not consecutively, but leaving time between the different examinations that the mind might come fresh to the subject-in the same manner as I have worked out other monographs which have lately appeared. There is not much inducement to bestow this labour on the groups ; for no sooner does the result appear, than some tyro in zoological studies, probably more a sportsman than a zoologist, who has shot and measured a few animals, comes to the Museum, casually inspecting the specimens, sometimes overlooking the most important of them, and gives his opinion, ex cathedra, on what he considers the distinctions of the species or their synonyma ; and unfortunately the compilers who come after the working zoologist, regard all the writers as of equal authority, and thus throw back the progress of science. c. Nose, forehead, cheeks, and chin covered with reflexed hair, the back, which is shorter and bent forward over the nose. 6. ARCTOPITHECUS CASTANEICEPS. (Plate XXXV.) Fur rather elongate and flaccid, blackish grey ; hinder part of the |