76 ANATOMY OF THE WOOD RAT 



specimens of Homodontomys in which this point was in- 

 vestigated this muscle arose by two heads from the ischium 

 and fascia in a single specimen, by a single head from the 

 fascia in one instance, and a single head from the ischium 

 in the other two. 



In Neotoma, origin was by two heads, the one from the 

 ischium being much the smaller, in one specimen, and by a 

 single head from the fascia in two cases. In three speci- 

 mens of Teonoma origin was by one head from the ischium 

 only. In this subgenus the insertional end is located a 

 trifle farther craniad, so that by its position the insertional 

 portion of the adductor femoris is completely separated 

 into two slips. 



M. semimembranosus posticus (figs. 6, 7, 32, 33) is 

 much the more robust of the two, and arises from the 

 superior tuberosity of the ischium and the adjoining caudal 

 border of that bone. It passes to the medial side of the 

 lower leg and is inserted by aponeurosis onto the tibial 

 collateral ligament and the medial tuberosity of the tibia. 



In Teonoma it is more robust and composed of coarser 

 fibers, being readily divisible into numerous layers. 



M. semitendinosus (figs. 6, 7, 32, 33) arises by two 

 heads, the more dorsal or lateral one of which has origin 

 from the fascia overlying the first two caudal vertebrae, 

 rather than from the vertebrae proper. The much smaller 

 ventral head arises from the tuberosity of the ischium 

 partly superficial to the origin of the biceps posticus. The 

 two heads shortly unite, the resulting muscle passing to the 

 medial side of the calf to an insertion upon the superficial 

 fascia. 



The biceps femoris occurs in two divisions, distinct 

 throughout their length. 



M. biceps femoris anticus (figs. 6, 32) arises from the 

 diapophyses apparently of the first two caudal vertebrae 

 just deep to the origin of the semitendinosus, and extends 

 to a narrow insertion upon the lateral border of the patella. 



