380 



THE EOCENE PEIUOD. 



Phijlof/oiif. — The serial arrangement of tlie bones of the carpus and 

 tarsus seen in the Tajeojmfa, is probably tlie primitive one, and we may 

 expect numerous accessions to the order on further exploration of the early 

 Eocene epochs. The modification seen in the more modern orders of Ppm- 

 ;•"■•; sodactyla and ArtiodadyJn, may Iju regarded as a rotation to the 



inner side, of the Imnes of the second carpal low, on those of 

 tliL' first. This rotation is probably nt-arly coincident with the 

 loss of the i)ollex, as it throws the weight one digit outwards, 

 that is on the third and fourth digits, rendering the first func- 

 tionally useless to a foot constructed solely for sustaining a 

 weight in motion. The alternation of the two rows of car- 

 pals clearly gives greater strength to the foot than their serial 

 arrangement, and this may probably account for the survival 

 of the tyi>e possessing it, and the extinction of nearly all the 

 species of the type which does not possess it. Here is aj)plied 

 again the principle first observed by Kowalevsky in the prox- 

 imal metapodial articulations. This author shows that the 

 i ) tj-pes in which the metapodials articulate with two carpal or 

 tarsal bones, have survived, while those in which the articu- 

 lation is made with a single carpal or tarsal have become 

 extinct. The double articulation is, of course, mechanically 

 Q V Cj8 the more secure against dislocation or fracture. 

 " f r As regards the inner part of the man us 1 know of no genus 



FiG.20.— Hind .^^.j^jp]^ presents a type of carpus intermediate between that of the 



foot of Poebro- * ' * ' 



tiie><um labia- Toxcopodd RXid Aiiihli/poda ou the one hand, and the Diplarthra 

 Ai'io'ut onf-'t"b'ir!i <^>'i tl'G Other. Such will, however, probably be discovered, 

 uatiirai size. ^^^ ^|,g earliest Permodactyla, n>i for m^tance Hi/racotherii())i, 

 Hyrachyus, and Triphpus, possess the carpus of the later forms, Rhinocerus 

 and Tapirus. The order Amhiypoda occupies an interesting position be- 

 tween the Taxeopoda and Diplarthra, for while it has the carpus of 

 the jn-imitive type, it has the tarsus of the later order. The bones of the 

 tarsus alternate, thus .showing a decided advance on tlie Taxtopoda. This 

 order is then less primitive than the latter, although in the form of its astrag- 

 alus it no doubt retains some primitive peculiarities which none of thd 



