AAA DR. J. E. GRAY ON THE BRADYPODID.E. [May 2, 



These three species with a spotted back and frontal band of soft 

 white hair are very much alike externally, though they have differ- 

 ences which are not easily expressed in words ; but perhaps they 

 would be more easily denned 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 cha- 

 racter that I pointed out in my former paper published in the ' Pro- 

 ceedings ' of 1849. That this character is permanent and not unim- 

 portant 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, how- 

 ever, form any ground for believing them to be variations of the 

 same form or species. The difference of form appears to be con- 

 stant 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 inter- 

 mediate 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 maimer 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 in- 

 specting 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 rejlexed hair, like 

 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 



