80 MR. R. LYDEKKER ON A SIRENIAN JAW [Feb. 2, 



two such ridges ; wliile in front it lias a long and prominent talon, 

 and behind a shorter one, whicb 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 Pro- 

 fessor 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 selenodoiit structure. 



In order to illustrate this I have had figured a left upper molar 

 (fig. 2, p. 79) of the extinct Siwalik genus Merycopotanius, 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-molar carry a 

 median ridge (c) flanked by two lateral ridges (a, b). 



Now if the metacone of the hinder tooth in our specimen be com- 

 pared with that of the Merycopotamus-mo\?ir, 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. Inconsequence 

 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 protocone ^. 



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 tliem clearly in a figure. 



