198 ANATOMY OF THE WOOD RAT 



DISCUSSION OF SUBGENERIC VARIATION 



A tabular comparison (table 1) is offered of those muscles 

 which exhibit relative differences between the three sub- 

 genera. Obviously it is practical to indicate these only in a 

 general way. In addition it must be borne in mind that a 

 number of them may be apparent rather than real, for the 

 specimens of Teonoma dissected were much larger than the 

 others and nice decision of an impossible degree of exactitude 

 was often necessary in order properly to decide whether a 

 myological difference encountered was relative or due merely 

 to the discrepancy in size between the individuals compared. 



In addition, there are differences in the degree of ease 

 with which certain muscles are separable from their neigh- 

 bors, as well as others which as far as we can see have no 

 functional variation, as in the precise conformation of such 

 muscles as the rectus abdominis, mentioned under the 

 proper headings. 



It will be noted from the table that if Homodon- 

 tomys be arbitrarily considered as the normal type with 

 which to compare the other two subgenera, Teonoma Ues 

 at the other extreme, on the whole showing the widest 

 departure in characters, while Neotoma occupies a position 

 between the two. The last is either intermediate or 

 resembles one or other of the remaining subgenera in 42 

 of the 48 myological points in which there is subgeneric 

 variation. In 24 cases it is nearer Homodontomys, in 13 

 nearer Teonoma, while in 5 instances it is fairly intermediate 

 between the two. In but 6 cases out of these 48 is Neotoma 

 more extreme in any particular than the two others. The 

 platysma, rhomboideus anterior, pectoralis abdominalis, and 

 sternocostalis are smallest or weakest in this subgenus, the 

 panniculus carnosus is heaviest, while the two bellies of the 

 digastricus are most distinct. In none of these 48 is the 

 development the same in Homodontomys as in Teonoma. 



Most of the differences indicated are but slight, and in 



