378 



E. W. WILLETT ON A MAMMALIAN JAW FROM 



the exception of the foremost, one are quite perfect. The genus to 

 which the specimen belongs is Triconodon, Owen; but it differs 



Pig. Lower jaw of Triconodon mordax, Owen, from Swanage. 

 Nat. size. 



from those described by Prof. Owen in his Monograph on the 

 Mesozoic Mammalia * in the fact that it has four teeth having the 

 form of true molars, all those previously found (eleven or twelve in 

 number) having each only three true molars, " a reduction rare in 

 the Marsupial order," to which these mammals are usually ascribed. 

 It may be noticed here that the nearly allied genus Triacanthodon 

 has four true molars, but that the teeth now under consideration 

 differ from those of this last-named genus in the following im- 

 portant details : — The fourth premolar of Triacanthodon approaches 

 the triconodont or true molar' type ; and the apex of this tooth 

 reaches only to half the height of the main cone of the preceding 

 premolar, whereas the main cone of the fourth premolar of the 

 new jaw is by far the largest of the three cones, and is rather 

 longer than the corresponding cone of the preceding tooth ; again, 

 no hinder talon is found in any of the four molars of Triacanthodon, 

 while it is well marked here on each tooth. 



Of the six teeth preserved, four are, judging by form, true molars, 

 and two are premolars, each of which is implanted by two roots ; 

 the two sockets for the next premolar are also plainly visible. 



The crowns of both the premolars consist of a principal sub- 

 compressed cone, with a small and low anterior basal cusp, and a 

 large and higher posterior one followed by a rudimentary talon ; 

 of the two the anterior tooth is rather the smaller. 



All the true molars agree with the description of the type 

 specimen of Prof. Owen in being " subcompressed, antero-pos- 

 teriorly extended, and divided into three nearly equal cones in the 

 same longitudinal line, the middle cone being very little larger than 

 the front or hind cone ; further, there is no cingulnm on the outer 

 side of the crown, but at the posterior margin of the posterior cone a 

 rudimentary talon is feebly marked off by a short vertical indent 

 from the rest of the surface of that cone." The first of the true 



* 'Monograph of the Fossil Mammalia of the Mesozoic Formations,' by 

 Prof. Owen, RE.S. DX L. Printed for the Palseontographical Society. 

 London 3871 



