404 THE KATITRAL HISTORY RETIEW. 



fossil species of the same genus : but though in many points they are 

 remarkably alike — such as in the development of the combing plates 

 (G- H) in the upper jaw, yet they present considerable points of dif- 

 ference. In the tichorhine milk molars, the thickness of the enamel 

 surface and its sculpturing, in the leptorhine the small size as com- 

 pared with the megarhine, afford a ready means of differentiation. 



B. megarliinus presents us with the same milk molar dentition as 

 the two other species, Dm 4 . 

 Dm 4. 



§ 3 A. Upper Milk Molars. — The posterior wall of the tooth 

 or the third collis (F) (= collis tertius of Brandt), in all the upper 

 milk molars is depressed in its middle part instead of bearing a cusp, 

 as in the tichorhine homologues. The grinding surface of the teeth 

 is more excavated by wear than in the tichorhine species, where it 

 is nearly flat. The fangs are four in number, the two outer free, the 

 two inner confluent. They are hollowed beneath by the pressure of 

 the germ of the successional tooth. 



The flrst tooth of the milk series in the upper jaw (Dm 1 : Figs. 

 1 and 2) is remarkable for its large size as compared with its homo- 

 logue in B. ticJiorhinus, The external surface or lamina, L of Pig. 

 1 and 2, is smooth, and with a regularly convex contour, both ver- 

 tical and horizontal, instead of being traversed by costse as in the 

 above species. The anterior valley. A, is wide, and traversed by two 

 small involutions of enamel. Anteriorly it communicates with the 

 exterior and anterior surface by a deep cleft descending almost to 

 the base of the crown, and insulating the anterior collis, D, from the 

 external lamina. It is smaller than the posterior vaUey B. There 

 is but faint trace of the development of * combing plates,' and con- 



riff. 1. 



sequently there is no accessory valley mapped off. At the base of 

 the cleft that separates the anterior collis, D, from' the lamina are 

 two small ridges, the one on the inner surface of the former, the 

 other on the outer surface of the latter. 



