1894.] MYOLOGY OF RODENTS. 289 



to show how far a femoral origin of this muscle is characteristic of 

 the Caviidae. 



In the Sciuromorpha the muscle is specially well developed and 

 encroaches on to the head of the fibula. There is no femoral head 

 except in Castor, but here it is not nearly as well developed as in 

 the Dasyproctidae and does not rise as in them by a definite tendon 

 {Sciiirus, Pteromys, Xerus, Spermopliilus, Arctomys, Castor). 



Extensor Lonyus Digitorum. — This muscle in all cases rises by a 

 tendon from the front of the external condyle just outside the 

 patellar surface on that bone. This is its only origin, except in 

 Spliingurus and Dipus, where a few accessory fibres rise from the 

 outer tuberosity of the tibia. The muscle passes down and becomes 

 tendinous in the lower part of the leg, being bound down by two 

 well-marked annular ligaments, the lower of which is attached to 

 the calcaneum and forms a distinct sling. 



The tendon divides for the four outer toes, when these are pre- 

 sent, being inserted into the middle and terminal phalanges. When 

 there are only three toes the middle one sometimes has a double 

 tendon, as in Lagostomus and Cavia cobaya. The tendons are 

 united by vincula, which in Myopotamus spread out in the web 

 between the toes ; they are strongly marked in Castor. 



Extensor Proprius Halluds. — This small muscle rises from the 

 middle or lower third of the front of the fibula and is inserted into 

 the terminal phalanx of the hallux, when that toe is present. In 

 Bipus cegyptius it is absent. In the Octodontidse it either unites 

 with or sends a slip to the extensor longus digitorum tendon to the 

 second toe (Aulacodus, Octodon, Myopotamus). In the Hystricidse 

 it rises from the lower part of the shaft of the fibula. 



In the Chinchillidse it is absent {OhincMlla, Lagostomus). 



In the Dasyproctidae it is a large muscle and rises from the 

 upper three-quarters of the fibula ; in Coelogenys it is inserted as 

 in the Octodontidse ; while in I)asyj)rocta, in which the hallux is 

 wanting, it goes entirely to the second toe, joining the long extensor 

 tendon there. 



In the CaviidsB it is present and runs to the second (internal) 

 toe ; it is bound down to the inner side of the base of the innermost 

 metatarsal by a short fibrous tunnel {Ceredon r-upestris, Cavia 

 cobaya, Bolichotis patagonica *). 



Among the Sciuromorpha it rises from the middle of the fibula 

 and is inserted only into the hallux in Sciurus, SpermopJiilus, Xerus, 

 and Arctomys. In Pteromys it springs from the lower third of the 

 bone and sends a small slip to the second toe. In Castor it rises 

 from a strong oblique fibrous band which runs from the head of 

 the fibula to the lower end of the tibia, so that the muscle has no 

 bony origin ; it rises opposite the middle of the fibula, and is 

 inserted into the first two toes. 



Extensor Brevis Bigitormn. — This muscle rises from the upper 

 and anterior part of the calcaneum, and runs to join the tendons of 



' Beddard, P. Z. S. 1891, p. 236. 



