1866.] ON THE ANATOMY OF THE CRESTED AGOUTI. 413 



the distal end of the elongated plantar surface of the naviculare), and 

 is inserted into the proximal end of the metatarsal of the index 

 (fig. 5, P. I.). 



In the Guinea-pig, Hare, and Rabbit it is similar ; but in one 

 specimen of the Guinea-pig the tendon passed altogether in front of 

 the malleolus. 



The peroneus quarti digiti (fig. 5, P. q. d.) is a much longer and 

 rather larger muscle than the preceding ; it arises by muscular fibre 

 from the outer side of the fibula, its whole length almost to the mal- 

 leolus. Its tendon goes beneath that of the peroneus longus, and is 

 inserted into the proximal phalanx of the outermost, i. e. fourth 

 digit. 



In the Guinea-pig, except that the origin is not quite so extensive, 

 it is similar ; but in the Hare and Rabbit its fleshy origin is much 

 shorter, it still arises, however, from the whole length of the much 

 shorter fibula. It is inserted also into the proximal phalanx of the 

 fourth digit. 



In the Crested Agouti and Guinea-pig there is no peroneus quinti 

 digiti present. In this respect they differ from the Common Rabbit, 

 where, according to Prof. Huxley * (whose observation we have veri- 

 fied), there are four peronei muscles, which he suggests may be, along 

 with the tibialis secundi, remnants of another set of extensors. 



In the Hare, where there are also four peronei muscles, the pero- 

 neus quinti digiti arises from the head of the fibula above the origin 

 of the peroneus quarti digiti. Passing behind the malleolus and be- 

 neath the tendon of the peroneus longus, it is inserted into the proxi- 

 mal phalanx of the fifth digit. 



The peroneus brevis is absent in Dasyprocta cristata. In the 

 Hare and Guinea-pig it is present, and arises from the upper end of 

 the anterior surfaces of both the tibia and the fibula, being inserted 

 into the outer side of the proximal end of the outermost metatarsal 

 (that is to say, the fifth) in the Hare ; but in the Guinea-pig with 

 a sesamoid bone beneath the proximal end of the outermost (*. e. 

 fourth) metatarsal. 



The peroneus tertius is absent in all the four animals compared ; 

 but in one specimen of the Guinea-pig the peroneus longus seemed, 

 as has been said, to assume to a certain extent the appearance of a 

 peroneus tertius, inasmuch as it passed in front of the malleolus. 



The gastrocnemius is very large, and has much the appearance of 

 the same muscle in man. It arises as usual and partly from two 

 sesamoid bones, that head which springs from the internal condyle 

 being slightly the larger of the two ; insertion the tuberosity of the 

 os calcis (figs. 4 & 5, Ga.). 



The Guinea-pig has also sesamoid bones to this muscle ; differing 

 in this respect from the Hare, in which we have found them absent. 

 In the latter animal the tendon of insertion in the os calcis is di- 

 stinct from that of the soleus. 



Plantaris. This arises by a strong tendon from the sesamoid 

 bone of the external condyle, and passing downwards beneath the 

 * Hunterian Lechires for 1864-65. 



