IMPACTS AND STRAINS ON" THE FEET OF MAMMALIA. 375 



ground, no sncli cross-strain would be experienced, and the effect 

 would be to consolidate by flattening the fixed articulation. 



III. The trochlege. These prominences, which form the 

 tongues of the tongue and groove articulations, exhibit various 

 degrees of development in the different Mammalia. Those of 

 different parts of the skeleton coincide in their 

 condition in any one type of ambulatory 31am- 

 malia, and so may be all considered together. 

 This fact suggests strongly that they are all 

 due to a common cause. 



They are all imperfect in the Rodcntia and 

 Carnivora (Figs. 75-77) (except the Leporidce, 

 which are especially characterized by their great 

 speed). Among ungulates they are very imper- 

 fect in the Prohoscidea. 

 The orders mentioned 

 all have elastic pads on 

 the under sides of their 

 feet or toes. The same 

 is true of the lowest 

 types of both the Ar- 

 tiodadyla and Perisso- 

 dactyla, the hippopo- 

 tami and rhinoceroses. 

 In the Rumina7itia the 

 trochleas are well de- 

 veloped (Fig. 80) with 

 one exception, and that 

 is the distal metacar- 

 pal and metatarsal 

 keels of the Camelidce 

 (Figs. 79, 81). These 

 animals confirm the 

 probability of the keels 

 being the effect of long- 

 continued shocks, for 

 they are the only Ruminants which have elastic pads on the in- 

 ferior sides of their digits. 



That these processes may be displacements due to shocks long 

 continued, is rendered probable by the structure of the bones 

 themselves. (1) They project mostly in the direction of gravity. 



Fio. T5. Fig. 76. Fio. 78. Fig. 77. 



Ficf. 75, Distal extremity of tibia of Amblyctonus 

 sinosvs Cope. Fig. 76, Distal extremity of tibia of 

 Oxyczna morsitans Cope. Both flesh-eaters, and two 

 thirds natural size. From Eeport Expl. and Surv. W. 

 of 100th Mer., G. M. Wheeler, iv, Pt. ii. 



Fig. 77, End of tibia and astragalus of ArcTicRlurus 

 dehilis. Fig. 78, Femur of JVimravusr/omphodus. Car- 

 nivora ; one third natural size. Mus. Cope. 



