4 



THE AMKRICAX XA TURALIST. [Vol. XXXV 1 1 1. 



digit V. In Pcrai;alc the latcml digits, except the hallux, are 

 subfunctional while in Choeropus the pes is functionally monodac- 

 tyl although not structurally so. The manus of Peragale has 

 five digits the three median ones being functional, with III as 

 the dominant finger, while the external digits are functionless. 

 The ungal phalanges on II, III, and IV are long and deeply 

 cleft. In Choeropus digits I and II are reversional, digit IV 

 being vestigial, while II and III are functional. III being as in 

 Peragale, the dominant one. 



From the digital modification observed in the Peramelidae to 

 that of the kangaroos is but a step, for while in the latter the 

 manus is more generalized the pes has reached a higher degree of 

 specialization in the total reversion of the hallux, the plan of modi- 

 fication being precisely as in the bandicoots. The more general- 

 ized pentadactyl manus, which shows no cursorial modification, 

 would seem to indicate that the bipedal gait was acquired before 

 speed requirements were met. 



The Rodentia have five or six families in which true speed adap- 

 tation occurs ; one, the Leporidae, which have a combination of 

 cursorial and saltatorial gait, the Dasyproctida? and Caviidae 

 which are purely cursorial, and the Dipodidae and Pedetid^ which 

 are saltatorial. 



In the Leporidae, the hares and rabbits, the gait is a curious 

 mixture of leaping and running. The wood rabbit, Lcpus sylva- 

 ticns, when in a full gallop progresses in a manner similar to that 

 of most quadrupeds, while the jack rabbit, Lcpus campcstris, 

 which is a true hare, moves by a series of bounds, irregular m 

 length, with all of the limbs moving synchronou.sly, though the 

 \veight borne upon the fore limbs must be very slight, the power- 

 ful hind limbs giving the impetus to the body. The Leporidae 

 show no especial digital modification other than the simple reduc- 

 tion of the hallux. There is an elongation and greater compact- 

 ness of the metapodium as in the Canidae among the Carnivora, 

 and the axis of the foot lies between digits III and IV. 



Of the purely cursorial types the Dasyproctidae are the more 

 generalized though they exhibit two distinct stages in the reduc- 

 tion of the digits. The first is that of Coelogenys, the paca, in 

 which the manus and pes are both structurally pentadactyl 



