DISCUSSION 213 



agility, of action, the degree being dependent upon the 

 completeness of such separation. These facts indicate a 

 thigh of more general power in Teonoma, with that of 

 Neotoma not appreciably more so than of Homodoniomys. 



There is indicated a tendency for Homodontomys to have 

 relatively the longest tibia (45 percent of the functional leg 

 length, while it is 44.5 per cent in Neotoma, and 43 per cent in 

 Teonoma). The relative lengths of the femur and tibia are 

 quite variable in Homodontomys. Usually the former is a 

 couple of millimeters the shorter, but occasionally it is a 

 trifle the longer; the average difference is not quite so great 

 in Neotoma while in Teonoma the two bones are practically 

 equal in length. In fact, in no case among the specimens 

 of the last available was the difference between these two 

 measurements greater than a half millimeter. No inter- 

 pretation of these facts is offered at present. 



Than Homodontomys the tibia is also lighter in Neotoma 

 and heavier in Teonoma. There are four other osteological 

 items of this bone that are best developed in the last 

 animal — the semimembranous process, popHteal fossa, the 

 lateral tibial fossa, and the groove upon the internal malleo- 

 lus. The last is probably of sHght significance. The 

 first, upon which process is inserted the semimembranosus 

 anticus, seems prominent in Teonoma merely because in 

 that animal the popUteal fossa adjoining is deeper. Its 

 pophteus muscle, whose chief function in the present genus 

 is not clear, was not noted as being larger, although it is 

 undoubtedly better developed in some way. The lateral 

 tibial fossa, shallower in Neotoma and deeper in Teonoma, 

 probably indicates sUght differences in the conformation 

 of the tibialis anticus. 



The head of the fibula is considerably the largest in 

 Teonoma, which may or may not mean that the peroneal 

 group of muscles arising therefrom is more efficient. The 

 distance from the head of the fibula to the point where it 



