104 DR. B. A. BEXSLEY ON THE EVOLUTION 



antei'ior and posterior ones, we first notice the fact that as regards contour a roughly 

 trianguhir shape predominates, and that, further, the apes of the triangle is internal and 

 is formed of a large cusp [l^r.), identifiable with the protocone in normal forms. In 

 some cases the protocoue-like cusp is well developed, but the triangular shape is 

 obliterated by the great development of the more posteriorly placed cusps. In one case 

 the protocone was found to be considerably elongated and its edge crenulated. The 

 anterior portion of the tooth is usually occupied by one internal {pa.) and two external 

 {ab and c) cusps. The anterior position of these elements is partially disturbed by the 

 occasionally anterior position of the protocone. The internal cusp, from its small size 

 and position, probably represents a paracone, while of the two external cusps the 

 anterior one, which stands in close relation with the paracone, is probably equivalent to 

 style ab of the Dasyurinse, and the more posterior one, judging from its hirge size and 

 position, is probably equivalent to style c. The paracone is often worn down and some- 

 times wholly absent. The posterior part of the tooth is usually occupied by two cusps, 

 one of which {me.), situate internally, is usually conspicuous for its large size. These 

 cusps probably represent together the metacone of normal forms. The reasons for this 

 view are : first, that their posterior position indicates a connection with the metacone ; 

 secondly, that they are sometimes connected by a trenchant edge {cf. fig. 5 a) ; and, 

 thirdly, that all of the well-developed cusps of the molars of Myrmecobius are elongated, 

 and that an elongation of such a cusp as the metacone of normal forms would very 

 likely result in the formation of an extra cusp from the distal extremity of its spur. It 

 must be admitted, however, that it is sometimes difficult to exj)lain the relations of 

 these elements on tliis supposition, on account of a reversal of their relative sizes, 

 although it is probable that the latter condition is due in some cases to the wearing to 

 which the internal cusps are subjected. 



As in the case of the lower teeth, the first molars are much simpler than the others. 

 Tiie best-developed specimen examined showed four cusps arranged in a linear series, 

 the tw^o intermediate members being much larger than the others ; and, in addition, two 

 minute cusps placed between the larger median elements, the one internally, the other 

 externally. The greatest departure from the condition in this specimen was found in 

 anotlier case, in which the tooth formed a flat plate, with no indication of cusps beyond 

 that arising from a slight concavity of the edge. The homologies of the cusps, even in 

 the better- developed teeth, are extremely doubtful, except that they show a general 

 rcsomblanoe, when seen in jjrofile, to the external styles in normal forms. 



The general conclusion with regard to the derivation of the upper molars is that they 

 represent modifications of a trituberculate type. The examination of a larger series of 

 specimens is, in the writer's opinion, necessary before this proposition can be accepted 

 as certain. 



Incisors. — All of the incisor characters of Ilynnecobms are directly derivable from 

 those of the Dasyurinae or Didelphyidse. The median upper teeth (text-fig. 2, a, b) are more 

 rounded in section than the lateral ones, and are also slightly procumbent, so that, while 

 they are not differentiated to the same extent as in normal insectivorous forms, they 



