478 LAND MAMMALS IN THE WESTERN HEMISPHERE 



The second family which was represented in Pampean times 

 was that of the jHegetotheriidae, and the sole genus of it which 

 survived so late was jPachyrukhos, a little creature no larger 

 than a rabbit. The genus went back without any noteworthy 

 change to the Santa Cruz stage of the Miocene, from which 

 complete skeletons have been obtained. The dental formula 

 was nearly as in ^Typotherium: i ^, c {], p f , 7W |, X 2 = 30, and 

 the enlarged, rootless and scalpriform incisors were similar. 

 The grinding teeth were thoroughly hypsodont and had a thin 

 coating of cement ; the molar-pattern was fundamentally 

 like that of ^Nesodon, in simpler form, but can be seen only in 

 freshly erupted and unworn teeth. 



The skull was very rodent-like in appearance, its flat top 

 and narrow, tapering facial region, and the gnawing incisors 

 adding much to the resemblance. The very large eye-sockets 

 and the enormously developed auditory region suggest noc- 

 turnal habits, and, no doubt, the timid, defenceless little crea- 

 tures hid themselves by day, perhaps in burrows. The en- 

 largement of the accessory auditory chambers, which all of 

 the fToxodontia possessed, reached its maximum in \Pachy- 

 rukhos, and the chambers formed great, inflated protuberances 

 at the postero-external angles of the skull. The neck was 

 short, the body long and the tail very short, much like that of a 

 rabbit. Collar-bones were present, as they probably were in 

 all of the other members of the suborder fTypotheria, though 

 this has not been definitely ascertained in all cases. The limbs 

 were relatively long, especially the hind legs, and very slender ; 

 the bones of the fore-arm were separate, but those of the lower 

 leg w^ere coossified at both ends. The feet, which had four 

 digits each, were of unequal size, the posterior pair being much 

 longer than the anterior, and the hoofs were long, slender and 

 pointed, almost claw-like. The entire skeleton suggests a 

 leaping gait and its proportions and general appearance were 

 remarkably like those of a rabbit-skeleton. In the restoration 

 (Fig. 300, p. 639) Mr. Knight has followed these indications 



