290 me. f. g. paesons on the [Mar. 20 



the extensor longus on the dorsum of the toes. It is present in 

 all the Sciuromorpha and Hystrieomorpha except Dipus. In no 

 case was it seen to give a tendon to the hallux, but tendons to the 

 second and third toes were always met with. In Coelogenys, Arcto- 

 mys, and Erethizon dorsatus l it sends a small slip to the fourth toe, 

 but it does not follow that when the four outer toes are well de- 

 veloped a tendon goes to each ; for in Octodon, in which all the five 

 toes are present, the muscle only sends slips to the second and 

 third. 



Peroneus Longus. — The peroneus longus showed very little vari- 

 ation in the Rodents examined. It rises from the head and the 

 upper part of the outer surface of the shaft of the fibula, also in 

 many cases by a few fibres from the external lateral ligament of the 

 knee. Its tendon passes through a groove on the outer side of the 

 external malleolus, grooves the cuboid, and is inserted into the 

 base of the first metatarsal bone, or, when that is absent, into the 

 second. In one specimen of Hystrir I failed to find it, but it was 

 present in another which I looked at. 



Peroneus Brevis. — The peroneus brevis rises either from the upper 

 or middle portions of the outer surface of the fibula. Its exact 

 origin is very variable and is not constant for animals of the same 

 group. It passes behind the external malleolus and is inserted 

 into the base of the fifth metatarsal bone. 



In Dipus cegyptius it is absent. 



In the Octodontida?, Hystricidae, and Chinchillidae it is present. 



In the Dasyproctidse it is absent in Dasyjwocta, but present in 

 Coelogenys. 



In the Caviidse it is present in Cavia cobaya and Ceredon, but 

 absent in Dolichotis 2 . In the two former it is inserted into a nodule 

 of bone (rudimentary fifth metatarsal ?) under the base of the 

 fourth metatarsal. In one specimen of Guinea-pig I found it 

 dividing into two parts ; the anterior, which was the smaller, had 

 the usual insertion, while the posterior was attached to the 

 anterior and upper part of the external surface of the calcaneum. 

 Beddard describes a somewhat similar arrangement in the peroneus 

 quarti digiti of DolicJiotis 2 . 



The muscle is present and normal in all the Sciuromorpha. 



Peroneus Quarti Digiti. — This muscle arises from the lower part 

 of the outer surface of the fibula below the origin of the peroneus 

 quinti, when that muscle is present. When the p. quinti is absent 

 the p. quarti rises from the upper part of the outer surface of the 

 fibula. The insertion is into the extensor tendon on the dorsum 

 of the fourth toe. 



In Dipus cegyptius it is present and rises from just below the 

 p. longus. In the Octodontida? it rises from the lower part of 

 the fibula {Octodon, Myoj)otamus, Capromys, Aidacodus). 



In the Hystricida? it is present in Ilystr'uv, but absent in Sphiiir 



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



2 P. Z. S. 1891, p. 236. 



