MAY, 1921. AMERICAN MARSUPIAL, C^NOLESTES OSGOOD. 131 



assumption would be that the increase took place before degeneration 

 of individual teeth began and for a different reason. The process of 

 reduction now beginning in Myrmecobius may perhaps be compared to 

 the more advanced condition in Tarsipes which retains but three or at 

 most four post-canine teeth and is evidently well on the way to an 

 edentulous state. 



Gidley makes the unequivocal statement that "There is in our 

 present knowledge nothing to support Owen's hypothesis regarding the 

 derivation of Myrmecobius." But in succeeding pages of the same paper 

 he argues for an ancient origin of the Myrmecobiidae, which he supposes 

 to have been well differentiated from the didelphids and dasyurids as 

 early as the Paleocene. Evidently he believes in a Mesozoic ancestor for 

 Myrmecobius and to that extent at least agrees with Owen. Whatever 

 the case may have been with respect to the history of the extra molar 

 teeth of Myrmecobius, the view that all the living families of marsupials 

 were well differentiated early in the Tertigry seems to be well founded. 

 Therefore, without reference to possibly archaic characters other than 

 teeth, it is still possible to believe in an early predidelphid origin for 

 Myrmecobius. As a possible forerunner of diprotodonts, the Myrme- 

 cobius line is thus entitled to some consideration. It has the same 

 antemolar formula as C&nolestes, its upper canines are somewhat 

 reduced, and its median lower incisors are greatly specialized and 

 relatively larger than in any other polyprotodont. These incisors are 

 completely terminal in position and, although slightly recurved at 

 the tips, are essentially proclivous with their roots nearly in the principal 

 longitudinal axis of the mandible and set at a decidedly different angle 

 from the lateral incisors. Thus the median lower incisors of Myrme- 

 cobius are essentially like those of diprotodonts. If these teeth are as 

 much reduced as the rest of the dentition, they must have been derived 

 from a type which might easily have led to the diprotodonts. The 

 median upper incisors of Myrmecobius are reduced in size and slightly 

 smaller than the lateral ones. They are set very far apart and are 

 slightly procumbent, obviously in a secondary condition which differs 

 from that of polyprotodonts and diprotodonts alike. Hence no con- 

 clusion is to be drawn from them although it is to be noted that the 

 median upper incisors are widely separated at the base in diprotodonts 

 more generally than in polyprotodonts. The first pair of upper lateral 

 incisors are modified to meet the median lower ones as in diprotodonts 

 and all the lateral incisors are laterally compressed. The differentiation 

 of the median upper incisors is scarcely to be regarded as a diprotodont 

 character for it is not pronounced except in a few extreme cases, as 

 Phascolarctos and Phascolomys, and in many it is distinctly less than in 



