212 ANATOMY OF THE WOOD RAT 



brae in relation to the mass of the animal is greater does 

 seem eminently logical, for such a development is not only 

 in line with what would be expected, but is indicated by the 

 sternum and clavicle being shorter in this animal in relation 

 to body length. 



A comparison of the functional arm length relative to 

 body length, and of the functional leg, measured by the 

 same standard, shows that in Homodontomys the arm is 

 66.8 per cent of the leg length, 65.7 in Neotoma, and 65 

 per cent in Teonoma. This variation is really too small to 

 be of decided import, although it should be noted that in 

 Homodontomys this is in the direction which is supposed to 

 indicate scansorial adaptation. At the same time, attention 

 should be called to the fact that there are numerous sorts 

 of scansorial and arboreal specializations of the limbs cor- 

 responding to differences in life habits, and it is extremely 

 easy for one to become misleadingly loose in his employ- 

 ment of such terms. This, however, is a matter for treat- 

 ment in detail in future papers. 



The length of the femur relative to the functional leg 

 length is 42 per cent in Homodontomys and Neotoma, and 43.2 

 in Teonoma. The only osteological detail of this bone show- 

 ing subgeneric variation is the lesser trochanter, which is 

 larger in Teonoma; but during dissection there was no 

 difference noted in any muscle inserting upon it. There are, 

 however, five other muscles of the femur exhibiting varia- 

 tion, all of them larger in Teonoma, and but one, which is the 

 gluteus maximus, equally as large in Neotoma. Of these 

 the gluteus superficialis anterior is used in flexion; the gemel- 

 lus superior and gluteus maximus, partly, in rotation; the 

 gracilis in adduction; and the gluteus maximus partly, in 

 abduction. In addition, the insertional separation into 

 two slips of the adductor magnus — used in extension and 

 adduction — which is partial in Neotoma and complete in 

 Teonoma, probably indicates increased efficiency, or at least 



