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Show 490 DR. R. BROOM ON THE STRUCTURE [Apr. 19, 5. On the Structure of the Theriodont Mandible, and on its Mode of Articulation with the Skull. By R. B R O O M, M.D., B.Sc, O.M.Z.S., Victoria College, Stellenbosch, S. Africa. [Received March 25, 1904.] (Plate XXXV.* and Text-figure 100.) As there are very strong reasons for believing that all mammals are descended from a Theriodont ancestor, all new light on the structure of the Theriodonts must be welcomed by morphologists. Only a very limited number of good Theriodont skulls have been discovered, and these have almost all been already described by Prof. Seeley. The majority of the types are in London, where they are readily accessible to European and American students, but in the museum at Grahamstown, S. Africa, there are two or three important specimens that seem worthy of more minute study than has yet been given to them. Recently, through the kindness of Dr. Schonland, I made a careful study of the Grahamstown specimens, with a view to getting more definite information about the structure of the lower jaw and its mode of articulation. Perhaps the most valuable specimen in the collection is the skull which forms the type of Cynognathus platyceps. It is an almost perfect skull, slightly crushed, and wanting only the snout. The mandibles are in position and the matrix has been beautifully cleared away. The specimen has been figured by Prof. Seeley and described at considerable length (1). The figure of the under surface unfortunately has been executed in such a manner that the student cannot readily determine what is matrix and what is bone, while there are no letters given to indicate the position of the different elements. The mandible is formed mainly by the dentary bone, which is so largely developed that when the jaw is viewed from the outer side all the other elements are completely hidden except in the articular region. The dentary has a very large coronoid process, which extends almost as far back as the plane of the articular region. The fractured anterior part of the bone shows the small splenial lyino-in a shallow groove on the inner side of the bone. Posteriorly the dentary received ori its inner side the angular and articular bones. The surangular lies on the inner side of the bone near the base of the coronoid process. The specimen does not show very satisfactorily the structure of the posterior part of the dentary. The articular is fairly well seen in the specimen. It forms a broad articulation with the quadrate and possibly with a small * For explanation of the Plate, see p. 498. |