THK MAMMALIA OF TIIK UINTA FOKMATION. 5G5 



8. Reduction and disj)lacement. There is unqucstiuii:il)ly an intimatL' c':)nncoti(ni 

 between the reduetioii ol" tlio latei'al toes and displacement. Kydci* has poiiitod out 

 that where the strains are ctjnally distributed among- all the digits, tiiere is rarely any 

 specialization of toes, and that in general the reduction of the digits is in direct ratio 

 to the increase of mechanical strains and the frequency and severity of impacts. 



Copet has recently assigned displacement as a cause or as preceding digital 

 reduction. This supposition is rendeied improbable by the numerous instances of 

 reduction without displacement, for instance, the first digit of the manus and fii-st 

 and fifth digits of the pes in ITyrax ; secondly, by the numerous cases of extreme 

 reduction, as in the Pecora, with limited disi)lacement. 



Xow if we consider the manus wilh reference to the correlation of certain types 

 of reduction and displacement, without reference to phylogeny, we find six types: 



I. The first type is theoretical, and will be found among tiie Puerco Condylar- 

 thra. In it there is no reduction or displacement. J Plieaacodus is the nearest known 

 representative, in which, however, there is initial reduction, metapodial displacement, 

 slight intercarpal displacement, and a marked tendency to the niesaxonic type. J I. 

 The second type embraces the Amblypoda, witli the lunar strongly and the scaphoid 

 slightly or not at all displaced. There is no distinct median axis. III. The third 

 or proboscidian type ditlers from the last only in the direction of llie lunar dis- 

 placement, which is usually upon the trapezoid. The median axis is not wei.1 defined. 

 IV. The fourth type is shown in a typical condition in Titanot/ier/'iai/, also more or 

 less marked in all tetradactyl Perissodactyla, as well as in all tetradactyl and 

 didactyl Artiodactyla ; it represents a stage in which the pcrissodactyl loot 

 approaches the artiodactyl, the thiid and fourth toes spreading upon either side 

 of the mesaxial line; the displacement of the caipals is apparently arrested, 

 the lunar retaining some foothold upon the magnum. Ail tridactyl and monodactyl 

 ungulates pass through this stage. Y. The fifth type is invariably associated with 

 tridactylism, incipient {Jfijraclnjns) or advanced (A2)lielops), in which displacement 

 is carried to an extreme and the mesaxial line jjasses directly through the third digit. 

 VI. The sixth is associated with monodactylisni and entire loss of lateral motion, in 

 which the primitive displacement is counteracted l»y the growth dI' l iif magnum. 

 \\\. The seventh is represented in Macraucheni'i, the lunar showing extreme reduc- 

 tion, both the scaphoid and unciform articulating with the magnum. 



* On llic Laws of Digilnl Hr'tliK'iion. AmcrirAn Naliinilisl, 1X77, p. 00;t. 



f "This rolaiimi lins resulted, sooner or lulcr, in the loss of the first <liglt." Aini-ricin Niitumlist. 18M0, p. Oho. 

 J The pes of I'uriptyfhui is so closely similar to that or the bear. Oii! nmniis may be assumed to I"' i>l' il"' -amc 

 type. 



A. P. S, vol.. XVI. .'Jt. 



