DISCUSSION 205 



must turn for most broad generalities, expressed by the 

 relational development of the bones, one with another. 



The skin muscles of the head and the platysma are best 

 developed in Teonoma, indicating superior mobility of the 

 integument of this region. Probably correlated with this 

 are the larger vibrissae and the importance of these tactile 

 organs in the life of this animal, while travelling the sub- 

 terranean intricacies of its native rock sHdes. Perhaps 

 in compensation is the fact that the panniculus carnosus^ 

 moving the body covering, is least developed in this sub- 

 genus; but why it should be heaviest in Neotoma is unknown. 



The slight differences in the masseter superficialis as 

 mentioned in the text but not in the tables are probably 

 without appreciable functional difference. Due to its 

 position, the masseter as a whole is not well situated to 

 exhibit any variation in mass which it may have. The 

 fact that the width of the anterior portion of the zygoma is 

 least in Teonoma, and that in this subgenus the bony septum 

 formed by the maxillary root of the zygomatic arch is small- 

 est, indicates, however, that the masseter is weakest in that 

 animal, slightly stronger in Neotoma, and strongest in 

 Homodontomys. In rodents the masseter is the muscle of 

 chief use in gnawing with the incisors, and therefore it is 

 permissable to infer that Homodontomys is the one most 

 given to such activities as piercing nuts with hard shells. 



Similarly, the greater width of the posterior portion of the 

 zygoma in Teonoma, coupled with its heavier temporal 

 ridging and temporal fossae, and longer coronoid processes 

 upon which the temporal muscles are largely inserted, 

 indicates that the latter muscles are most powerful in this 

 animal, less so in Neotoma, and least in Homodontomys. 

 This is corroborated by an examination of the muscles them- 

 selves. The temporals are the muscles principally used in 

 grinding the food with the molars, and this strengthens the 

 contention already advanced that Teonoma is given to the 



