286 me. f. g. parsoxs ox the [Mar. 20, 



the ischium ; the insertion i9 normal, but an expansion is continued 

 forwards deep to the internal lateral ligament of the knee. 



The oblique slip in Myopotamus and Octodon rises from the 

 tuber ischii, but in Aulacodus it comes from the sacral vertebrae 

 as in Hystrix (Aulacodus, Capromys melanurus l , Myopotamus, 

 Octodon). 



In the Hystricidae the arrangement is not constant : Hystrix and 

 Erethizou entirely resemble Aulacodus in the origin of the two parts, 

 but in Sphingurus the second slip rises from the tuber isc-Lii with 

 the rest of the muscle instead of from the sacral vertebrae ; it soon 

 becomes distinct to run to the internal condyle. 



Among the Chinchillidae, Lagostomus has the same arrangement 

 as Hystrix and Aulacodus, but Chinchilla resembles Sphingurus, 



In the Dasyproctidae the two parts of the muscle rise together 

 from the tuber and ramus and only separate towards the lower 

 part of the thigh {Dasyprocta, Cceloyeays). 



In the Sciuromorpha the condylar portion of the muscle is sepa- 

 rate from the rest and closely connected to the adductor mass, 

 with which it will for convenience be described ; it is, however, still 

 supplied by the great sciatic nerve (Sciarus, Pteromys, Xerus, Sper- 

 mophihiS, Arctomys, Castor). 



It will be seen that the Rodents illustrate the changes by which 

 part of the semimembranosus becomes part of the adductor magnus. 



In Godogmys, for example, the slip going to the condyle of the 

 femur is part of the semimembranosus : in Sphingurus it rise* with 

 that muscle but soon separates from it ; in Hystrix it is a perfectly 

 distinct muscle, having a different origin to the semimembranosus 

 or adductors, while in Sciurus it is intimately blended with the 

 adductors, though still preserving its original nerve supply. It is 

 remarkable, too, that, except for its constant arrangement in the 

 Sciuromorpha, it seems to be of very little classificatory value. 



Gracilis. — The gracilis is very often double. The most anterior 

 portion, which has sometimes been described as the sartorius, rises 

 from the ilio-pectineal line and from the anterior part of the 

 ventral surface of the symphysis pubis ; it is inserted into the inner 

 border of the patella and ligamentum patellae. The posterior 

 gracilis usually rises from the posterior part of the ventral surface 

 of the symphysis and from the subpubic arch ; its insertion is into 

 the cnemial crest of the tibia? and the fascia of the leg, which makes 

 it a powerful internal rotator of the tibia?. The two muscles are 

 always supplied by the obturator nerve. 



Sometimes the gracilis is a single muscle, but it then usually shows 

 signs of a separation. 



The general rule seems to be that the Hystricomorpha have two 

 graciles and the Sciuromorpha one. The two exceptions that I 

 have met with are Aulacodus, in which the muscle is single, and 

 Castor, in which it is double. 



Pectineus. — The pectineus rises from the ilio-pectineal eminence 

 and line under cover of the anterior gracilis, and is inserted into 

 1 P. Z. S. 1884, p. 234. 



