1894.] MYOLOGY OF RODENTS. 291 



gurus, and presumably in Erethizon dorsatus and epixanthus, as it is 

 not mentioned by Mivart l or Windle 2 . 



In the Chinchillidae it rises from the upper part of the fibula, 

 the p. quinti being absent {Chinchilla, Lagostomus). 



In the Dasyproctidae it rises from the whole outer surface of the 

 fibula in Dasyprocta, in which there is no p. quinti ; in Ccelogenys 

 it only rises from the lower third of the bone. 



In the Caviidae it resembles the Chinchillidae in Ceredon and 

 Cavia cobaya. Beddard mentions that it is present in Dolichotis. 



It is always present in the Sciuromorpha, having the usual 

 attachments. 



In Sciurus, Pteromys, Xerus, and Spermophilus it rises from the 

 lower quarter of the fibula and runs to the fourth digit. 



In Arctomys marmotta it sent an additional slip to the third toe. 



In Castor it was joined by a small muscular slip from the calca- 

 neum, probably part of the extensor brevis digitorum. 



It will be noticed that the only animals in which this muscle 

 was wanting were the Tree-Porcupines. 



Peroneus Quinti Digiti. — The p. quinti when it is present rises 

 from the outer surface of the fibula above the last muscle, and is 

 inserted into the extensor longus tendon on the dorsum of the 

 fifth toe. It is present in the Octodontida? {Myopotamus, Capro- 

 mys, Octodon, Aidacodus), in the Hystricidae {Hystrix, Sphhtgurus, 

 Erethizon), in Ccelogenys, and in all the Sciuromorpha examined. 



It is absent in Dipus cegyptius, in the Chinchillidae {Chinchilla, 

 Lagostomus), in the Caviidae {Cavia cobaya, Ceredon, Dolichotis), 

 and in Dasyprocta. 



The presence or absence of the p. quinti seems to depend entirely 

 on the degree of development of the fifth toe. It is not nearly as 

 persistent a muscle as the extensor proprius hallucis, which is 

 so often found when no hallux exists ; it seems indeed to precede 

 the disappearance of its toe, because in Chinchilla the muscle is 

 wanting, although there is a small fifth toe. 



Gastrocnemius. — The gastrocnemius rises by two heads from the 

 upper and back part of the two condyles, fabellae often being 

 present. The two heads unite with the soleus to form the tendo 

 Achillis. The fibres of this tendon are twisted so that those that 

 are derived from the inner head of the gastrocnemius become 

 superficial and eventually external. In Castor canadensis the two 

 heads remain separate as far as their insertion. 



The presence or absence of the fabellae does not seem to depend 

 on the affinities of the animal, as they are large in Aidacodus on 

 both sides, while in Myopotamus only the outer one is present. 

 In Dasyprocta they are both present, in Ccelogenys both absent. 



In the Sciuromorpha, however, they were found in every case 

 except that of Castor canadensis {Sciurus, Pteromys, Xerus, Sper- 

 mop)hilus, Arctomys, Castor). 



Soleus. — The soleus rises in all cases from the posterior aspect 



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



2 Journ. Auat. vol. xxii. p. 126. 



