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Show 1899.] ON THE PLEXOLOKT AlOLARS OF MAMMALS. 555 fringes 15. The left opercular gill was absent, and nothing indicated its absence to be the result of an injury. Mr. E. Lydekker, F.Z.S., exhibited a pale-coloured specimen of the Eeed-buck (Cervicapra arundinum), and read the following notes on it, extracted from a letter addressed to him by Mr. Ewarc S. Grogan:- "I have much pleasure in forwarding to you the horns, head-skin, and hide of what appears to be a white Eeed-buck. I shot the latter on the Longwe, at the north end of Lake Nyasa. Capt. Verhellen, of Mohun's expedition, first called my attention to it, by asking me (he knows nothing of the game in this part) what those little grey antelope were; he was very positive as to having seen four: one, a female, he wounded and lost; but though I hunted the small plain where he states he saw them, I never found any but the ram I killed, and it is the Eeed-buck's habit to generally run in the same party ; i. e. four running together would, I think, never go far apart, at any rate at the same season of the year. The natives whom I questioned closely say they have seen one only; but this counts for little. The buck showed no signs of albinism-lips, nostrils, eyes, and hoofs being of the normal colour. On comparing the skull with two others I thought I detected considerable variations, especially in the base of the skull. Will you kindly describe the animal for me, and bring it Isfore the notice of those who are interested in this branch of zoology ? Personally I am inclined, owing to the persistent rumours of similar animals in this country, the striking and very definite assertion of Capt. Verhellen, and the complete absence of the usual signs of albinism, to think that it is a distinct form. I have taken what measures I could to preserve the skin and trust that it will arrive in good order." The following papers were read :- 1. On the Primitive Type of the Plexodont Molars of Mammals. By FLORENTIO AMEGHINO, C.M.Z.S. [Eeceived February 13, 1899.] The majority of placental Mammals, and especially the Ungulates, are distinguished by the plexodont character of their molars-that is, by molars having a complicated crown, and each tooth being provided with more than one root. The origin of this dentition has been explained by two completely different theories-the theory of a gradual complication, and that of fusion. According to the former, the plexodont molars are the result of a progressive complication of the simple and conical primitive tooth of Eeptilia. According to the latter, these same teeth are the result of fusion of the dental germs or embryos of |