MAESUPIALIA. 791 



first place, there is an inherent improbability in the snpposition that the 

 external Vs of the superior molars of the Insedivora have had the same 

 origin as those of the Ungulata. Tlie movements of the jaws in the two 

 groups are different, the one being vertical, the other partially lateral. In 

 the one, acute apices are demanded; in the other, grinding faces and edges. 

 We have corresponding Vs in the inferior dental series, and we regard those 

 as produced by the connection of alternating cusps by oblique ridges. In 

 homologizing the superior cusps, we have, as elements, two external, two 

 intermediate, and two internal cusps. The first are opposite the external 

 roots, and the anterior internal is opposite the internal root. 



First, as regards Centetes and ChrysocMoris. Besides the strained char- 

 acter of the hypothesis that supposes the V-shaped summit of the crown to 

 represent two V-s fused together, there is good evidence obtainable in sup- 

 port of the belief that the triangle in question is the usual one presented by 

 the Creodonta. This clearly consists of the two external and the anterior 

 internal cusps united by angular ridges. The form is quite the same as in 

 Leptictis and IctojJS^ (Plate XXIX a, fig. 3), and nearly that of Deltatherium 

 (Plate XXIII d, fig. 8 a), where the external cusps are present. Centetes 

 and ChrysocMoris only differ from these in that the external cusps are 

 wanting. In addition, the latter genus presents a rudiment of the posterior 

 inner tubercle, as is seen in Deltatherium (Plate XXV a, fig. 10). An ex- 

 planation similar to this is admitted by Mr. Mivart to apply to the cusps of 

 the inferior molar of Centetes. It remains to ascertain whether the apex of 

 the V in ChrysocMoris represents the internal or intermediate cusp. 



Secondly, as regards the Talpidce and Soricidce, where the external Vs 

 are well marked. If we examine the external cusps in the genus Didel- 

 phys, we find that the posterior one becomes gradually more anterior in its 

 position, until on the second true molar it stands largely above the inter- 

 space between the roots, instead of over the posterior root. It will also be 

 seen that the anterior intermediate tubercle is distinct and of insignificant 

 proportions, while the posterior intermediate is large and is related to the 

 posterior external, as is the apex of a V to its anterior base. In this ar- 

 rangement I conceive that we have an explanation of the Vs of the Talpidce 



' Leidj's Extinct Faiina of Dakota and Nebraska, 351, PI. XXVI, fig. 29. 



