1894.] MYOLOur cur bodhhts. 293 



merit that I have seen was in Pteromys oral, in which the flexor 

 tibialis divided into two slips, one of which had the usual Seiuro- 

 morphine insertion below the internal cuneiform, while the other 

 joined the temlon of the flexor fibularis. Possibly this was 

 an individual variation foreshadowing the arrangement in the 

 Hystricoinorpha. 



Tibialis Posticus. — The description of the tibialis posticus is 

 included in that of the "Long Plexors of Rodents" by Dobson l . 

 In Castor it is inserted into an extra bone on the inner side of the 

 internal cuneiform. 



Lumbricales. — The number of the lumbricales seems to depend 

 on the number of toes ; thus all the Sciuromorpha and the Hystri- 

 comorpha possessing five toes, such as Myopotamus, have four 

 luinbricales. Coeloyenys, although it has five toes, has only three 

 lumbricales. Animals having three toes usually only possess two 

 lumbricales, e. g. Dasyprocta, Cavia cobaya, and Ceredon rupestris. 

 In Dolichotis Beddard only found one 2 , but in another specimen 

 which I had the opportunity of looking at there were two. 



Muscles of the Foot. 



The accessorius is absent in the Dipodidse and Caviidae, but 

 present in the other animals examined, including the Sciuromorpha. 

 It rises from the outer surface of the calcaneum, usually from the 

 anterior part, and is inserted into the plantar surface of the flexor 

 tendon just before it divides for the toes. The angle which it 

 forms with the flexor fibularis is a very open one, about 45°, but 

 in Hystrlv it must be about 70° or 80°. 



When the foot is well developed there are two interossei to 

 each metatarsal bone. When the hallux is well developed, as in 

 Myopotamus, the abductor hallucis rises from the sustentaculum 

 tali, or from the scaphoid, as in Octodon, but when it is not developed 

 the muscle is absent. In the Sciuromorpha the abductor minimi 

 digiti often rises from the calcaneum as well as from the base of 

 the fifth metatarsal ; in this case the part between the calcaneum 

 and the metatarsal will form an abductor ossis metatarsi quinti. 



On the plantar surface of the interossei there are frequently 

 found two muscles rising from the deep cartilage of the sole which 

 forms the sheath of the peroneus longus tendon ; from this they 

 run forwards, diverging from one another like the limbs of a V. 

 The inner of these is in some of the Sciuromorpha inserted into 

 the outer sesamoid bone under the head of the metatarsal bone of 

 the hallux, forming an adductor hallucis, but more often, as in 

 Myopotamus, Octodon, Uystruv, and Coeloyenys, it is attached to a 

 corresponding situation on the second toe forming an adductor 

 secundi digiti. The adductor minimi digiti (the outer of the two 

 muscles) is attached to the inner sesamoid bone of the little toe. 

 These two muscles are wanting in the Dipodidae and the Caviidae, 



1 Journ. Anat. vol. xvii. p. 159. 



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



Pboo. Zool. Soc— 1894, No. XX. 20 



