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Show 80 MR. R. LYDEKKER ON A SIRENIAN JAW [Feb. 2, two such ridges ; while in front it has a long and prominent talon, and behind a shorter one, which runs upwards to join the inner extremity of the second transverse ridge. In the inner half of the transverse median valley there is a small double tubercle. I shall have more to say on the structure of the ridges of this tooth by-and- by. The anterior one of the two teeth differs from the other in being much more elongated, and in narrowing gradually from back to front, where it terminates in a blunt point. It has, moreover, three, instead of two, transverse ridges; the second and third ridges corresponding closely in structure with the two ridges of the hinder tooth, while the first ridge is more irregular in shape, and has on its anterior surface a rudiment of the anterior talon of the hinder tooth. Reverting to the latter, an examination shows that the transverse ridges are not simple continuous ones, but are formed by an inner and outer column, closely pressed together; the whole four columns corresponding to those of an ordinary bunodont molar, like that of a Pig, and representing the proto-, para-, meta-, and hypocone of Professor Osborn's system of notation of these dental elements. A further examination will, moreover, show that these four columns are not of a bunodont type, but are modelled on a peculiar modification of the selenodont structure. In order to illustrate this I have had figured a left upper molar (fig. 2, p. 79) of the extinct Siwalik genus Merycopotamus, which is a specialized representative of the Artiodactyle Ungulate family Anthracotheriidce. It will be apparent from that figure that the external surfaces of the outer columns (paracone and metacone), instead of being nearly upright, are inclined very much towards the centre of the tooth, so that the summits of these columns are squeezed into the crescents formed by the inner columns (protocone and hypocone). It will further be observed that the external surfaces of the paracone and metacone of the Merycopotamus-xnolar carry a median ridge (c) flanked by two lateral ridges (a, b). Now if the metacone of the hinder tooth in our specimen be compared with that of the Merycopotamus-mo\ar, there cannot be any hesitation in recognizing their close similarity in structure-the external surfaces of the outer columns showing the same inclination towards the centre of the crown and having the same general contour. In the present specimen, however, the metacone has assumed a more oblique position to the axis of the crown, and has been squeezed right into the heart of the crescent of the hypocone. In consequence of this the postero-external ridge of the metacone has been lost, and its central and anterior points have come into line with the central cusp of the hypocone to form the imperfect transverse ridge of the molar. A precisely analogous condition obtains with regard to the paracone and protoconel. It appears, therefore, to be quite evident that the hinder tooth of the specimen under consideration is constructed on an extreme and 1 Although these points are apparent enough when the actual specimens are compared, it seems to be impossible to indicate them clearly in a figure. |