MR. W. B. DAWKINS ON EHINOCEROS MEGAEHINUS. 409 



The difference between Premolars three and four is one merely 

 of size. They are characterised by the tumidity of their external 



_JVr-V, ^^^ 



lamina, L, and by the absence of any trace of a third costa, that is 

 developed so persistently in their leptorhine homologues. The an- 

 terior valley. A, is traversed by irregular processes of enamel, as in 

 the teeth figured by M. de Christol. Its entrance is from 1.05 to 

 1.0 inches from the base. The ascending ridge or guard on the 

 anterior aspect sweeps round the inner base of the crown and ascends 

 the median collis. This is also the case with the leptorhine teeth of 

 Professor Owen, but the guard in the latter is less prominent, and 

 a glance at the enamel structure afi'ords a ready means of differen- 

 tiation. The leptorhine teeth moreover throughout are smaller than 

 the megarhine. 



The only points of difference between the true molars one and 

 two is, that in the former the stout ascending guard on the anterior 

 aspect extends inwardly as far as the middle of the inner base 

 of the anterior collis, D, in the former, while it never extends 

 so far in the latter. In the latter (Figs. 10, 11) also the posterior 

 lobe is, relatively, smaller in transverse measurement than the an- 

 terior. 



In both these teeth the second costa, K 2, is strongly developed 

 and is higher than the first. The median depression so constant in 

 the external lamina of the tichorhine homologues is absent, and the 

 posterior area in place of bearing costae presents a smooth and 

 gently waved contour, N. The entrance to the anterior valley is 

 wide in some, narrow in other teeth, at times it is blocked up by a 

 small cusp. The posterior combing plate, H, developed from the 

 anterior surface of the median collis is constant, and extending for- 

 wards partially insulates the head of the anterior valley. This, 

 rounded in some, trihedral, in others, is traversed by vertical folds of 

 enamel. The posterior valley is triangular in outline. The anterior 

 collis, D, is traversed anteriorly by a strongly developed guard that 

 circumscribes a deep pit on its inner and anterior base. The median 



