MAY, 1921. AMERICAN MARSUPIAL, C^ENOLESTES OSGOOD. 117 



with that on the other, a condition approached in no polyprotodonts 

 except Myrmecobius and Perameles. The second and third are in contact 

 with each other, the second being somewhat larger than the third. 

 They are laterally compressed into relatively broad blades, their cutting 

 edges are truncate and their bases narrow. The blades, which are dis- 

 tinctly hatchet-shaped, are somewhat produced anteriorly. These teeth, 

 although small, are completely functional since they engage the sharp, 

 beveled, outer edges of the median lower incisors with which they have a 

 very effective shearing action. Similar action is seen between the 

 incisors of some of the macropods, and especially of certain of the 

 Phalangeridae, as for example, Dactylopsila, in which it is very pro- 

 nounced. Somewhat similar relations occur in Perameles. 



The fourth upper incisor is abruptly smaller than the preceding ones 

 and is usually separated from them by a slight space. In superficial 

 view it appears to be pointed but closer examination shows that this is 

 due to a partial forward rotation which has elevated the posterior angle 

 of its cutting edge. This cutting edge is thus inclined forward so that 

 it is parallel to the unbeveled edge of the lower incisor and in this rela- 

 tion it is possible for it to be slightly functional. 



Discussion. All the upper incisors are definitely adapted for relation 

 with the long procumbent lower incisors. The result is the modification 

 of the lateral upper incisors from simple seizing or grasping teeth to 

 cutting or shearing blades. Such a modification, as shown byBensley,is 

 easily derived from the conical tooth by lateral compression accom- 

 panied by the movement of its anterior edge to a horizontal position and 

 its posterior edge to a vertical one continuous with the root. The 

 original apex of the tooth thus becomes the posterior angle. In less 

 marked degree this change has been noted and figured in Myrmecobius 

 and Perameles by Bensley (1903, p. 105) who speaks of it as a "curious 

 appearance, due to a subcaniniform modification of their tips," and 

 makes a rather obscure comparison with a condition in the degenerate 

 teeth of the Madagascan viverrine Eupleres. 



In viewing these teeth comparatively in Myrmecobius, Perameles, 

 and Ccsnolestes, one is at once impressed with the practical identity in 

 form and function in all three forms and it is evident that the modifica- 

 tion of the lateral upper incisors is correlated with the elongation and 

 specialization of the median lower incisors. In other words, this form 

 of lateral upper incisor is an indication of beginning diprotodonty and 

 is only found in the Australian diprotodonts and forms conceivably 

 ancestral to them. This subject is discussed elsewhere. It may only 

 be said further that the reciprocal relations of the median lower incisors 

 and the lateral upper ones are exceedingly well-developed in C&nolestes. 



