1896. ] MYOLOGY OF RODENTS. 179 
Sematendinosus.—The normal arrangement seems to be, as in the 
other Rodents, that one head should rise from the posterior sacral 
and anterior caudal vertebre, while the other comes from the tuber 
ischii. Either of these heads is often wanting. In Georychus, 
Bathyergus, and Mus barbarus the head from the tuberosity alone 
was found, while in Microtus and Heteromys only the spinal head 
was present. With regard to the arrangement in Mus barbarus, 
it is interesting to note that Mus rattus has both heads. The 
insertion in all cases is the same as that described in the other 
Rodents. 
Semimembranosus.—The two parts of this muscle are always 
present, and correspond to the description already given in other 
Rodents. The semimembranosus proper is quite constant, always 
rising from the tuber ischii and being inserted into the back of 
the internal tuberosity of the tibia. The supracondylar slip, as 
in the Hystricomorpha, is variable both in size and in origin; its 
insertion, however, above the internal condyle of the femur is 
quite constant. In Gerbillus, Mus barbarus, and Mus rattus the 
supra-condylar portion rises from the anterior caudal vertebrae. 
In Cricetus it is small and comes from the caudal vertebrae and the 
tuber. In the other animals examined it rises from the tuber 
ischii only in common with the rest of the semimembranosus 
except in the case of Buthyergus, where its origin is more in 
common with that of the semitendinosus from the tuber. This 
supra-condylar slip is largest in G'eorychus, where it is greater than 
the rest of the muscle; in Bathyergus it is not so large; in Rhizomys 
it is only half the size of the rest of the muscle; while in Cricetus 
it is quite small. In Hydromys,Windle says‘ that the semimem- 
branosus is inserted into the condyle of the femur alone, that is 
to say, the main part of the muscle is wanting. His dissection 
must of course be repeated, to see whether he had chanced upon 
an individual variation or whether this arrangement is constant 
in Hydromys. The nerve-supply of the two parts of the semi- 
membranosus is as in other Rodents. 
Gracilis—This muscle is usually double and the two parts have 
approximately the attachments described in the Hystricomorphine 
Rodents ; the anterior muscle usually overlaps the posterior a good 
deal. In Myoxus, Rhizomys, and Heteromys no separation was seen. 
In Georychus and Bathyergus the separation was very warked, 
there being quite an interval near the insertion. In Myodes the 
interval is greatest at the origin. Hydromys according to Windle 
agrees with Myoaus and Rhizomys. 
Pectineus.—This muscle corresponds to the account of it in the 
other Rodents. In Gerbillus, Cricetus, Myoxus, and Rhizomys the 
muscle is double, but I am inclined to regard the inner part as 
belonging to the adductor mass, possibly representing the adductor 
longus. 
Quadriceps Extensor.—In all the animals dissected, the two heads 
of the rectus could be made out, though in Rhizomys, Georychus, 
1 P.Z.S. 1887, p. 57 We 
