284 MB. F. G. PARSONS ON the [Mar. 20, 



The intra-pelvic portion seems often to be wanting, in which case 

 it is very difficult to define the muscle. 



In the Dipodidae it is continuous with the gluteus minimus 

 (Dipus wgypthis). 



Among the Octodontidse it is found between the two layers of 

 the folded gluteus minimus in Aulacodus and Capromys ; in Myo~ 

 potamus and Octodon it is normal. 



In the Hystricidae it was not seen in Hystrix, but was present 

 in Sphingurus. 



In the Chinchillidae it was not seen in Chinchilla, but was made 

 out with some difficulty in Viseacha. 



In the Dasyproctidae it is large and well marked, partly over- 

 lapping the gluteus medius (Dasyprocta, Ccelogenys). 



In the Caviidae it rises from the sacrum, but is continuous with 

 the posterior border of the gluteus minimus (Cavia cobaya, Ceredon 

 rupestris). 



In all the Sciuromorpha examined it was present and normal, 

 except in Castor, where it was not seen. 



Obturator Interims and Cemelli. — These muscles usually have 

 the same origin as in Man, but they are inserted into the digital 

 (trochanteric) fossa of the femur. The gemelli are large, especially 

 the anterior one. 



When the obturator internus is cut through a number of 

 tendons are seen on its deep surface converging to the digital 

 fossa ; this is well shown in Ccelogenys. In Castor canadensis the 

 origin of the obturator internus and gemellus posterior was taken 

 up by the great tail-muscles, so that the anterior gemellus was the 

 only part of this group present. 



Obturator Externus. — In all the Rodents dissected, this muscle 

 had the usual human attachments. Its insertion into the digital 

 fossa is deep to that of the obturator internus. 



Quadratus Femoris. — The quadratus femoris rises from the outer 

 side of the tuber ischii, and is inserted into the back of the femur 

 midway between the great and small trochanters. In the Hystri- 

 comorpha it is usually inserted by a narrow tendon, but in 

 SpJiingurus and the Sciuromorpha the insertion is fleshy and the 

 muscle quadrilateral. 



Biceps Femoris. — The biceps rises from the spines of the anterior 

 caudal vertebrae and the fascia over the tail-muscles ; also by a 

 dee]) head from the tuber ischii. These two parts, as a rule, unite 

 and are inserted into the outer side of the patella, and the fascia 

 of the leg from the knee almost, if not quite, to the ankle. 



This arrangement obtains in all the Rodents dissected, with the 

 following exceptions : — In Sphingurus the two parts remain 

 distinct, the superficial or caudal portion being inserted into the 

 outer side of the patella and ligamentum patellae, while the deep 

 portion from the tuber ischii is joined by a slip from the posterior 

 sacral vertebrae and is inserted into the fascia of the leg continuing 

 the plane of insertion of the former part. In Erethizon dorsatus l 

 1 P. Z. S. 1882, p. 271. 



