1894.] MTOLOGT OF EODES^TS. 291 



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

 not mentioned hy Mivart ^ or Windle ^. 



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

 the p. quinti being absent (ChinchiUa, Lagostomus). 



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

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

 it only rises from the lower third of the bone. 



In the Caviidae it resembles the Chinchillidae in Ceredon and 

 Cavia cohaya. Beddard mentions that it is present in Dolicliotis. 



It is always present in the Sciuromorpha, having the usual 

 attachments. 



In Sciurus, Pteromys, Xe7-^(s, and S2yernw2yhiliis it rises from the 

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



In Arctomys mamiotta 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 Octodontidae (Myopotamus, Capro- 

 mys, Octodon, Aulacodus), in the Hystricida? (Hystriv, Splnrujurus, 

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



It is absent in Dipus (xgyptins, in the Chinchillidae {Chinchilla, 

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

 and in Dasyjyrocta. 



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. 



Gastrocnenmis. — The gastrocnemius rises by two heads from the 

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

 present. The two heads unite vnth the soleus to form the teudo 

 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 fabellse 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 C(xlogenys both absent. 



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

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

 mopjhilus, Arctomys, Castor). 



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



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



2 Journ. Anat. vol. xsii. p. 126. 



