On the Osteology of the Hyopotamidge. 169 



nished by fossil remains and by lawful induction, what are the exact 

 modifications of the skeleton exhibited by each gi-oup along the 

 ascending and descending lines. As these modifications were most 

 clearly given by greater or less reduction of the manus and pes, I 

 subjected these to a detailed comparison. 



In tracing the Paridigitata in time, we cannot mistake the ten- 

 dency clearly manifested by them to a gradual reduction of the 

 manus and pes in such a way that each descendant is always some- 

 what more reduced than its immediate predecessor. The Kmbs in 

 the Ungulata serving only for the support of the body, and not for 

 prehension, the organism seems to derive a great advantage from 

 their reduction and simplification. 



By a comparative study of the least-reduced representatives on 

 both fines, I tried to ascertain the probable structure of the manus 

 and pes in the progenitor that has given rise to both groups, or to 

 the whole assemblage of Ungulata ; and this led me to construct 

 a typical manus and pes. On the correctness of this scheme we 

 may to a certain extent rely, as it is exhibited in nearly all its details 

 by the living Hippopotamus, the most complete form of the living, 

 and by the Hyopotamus and Anthracotherium, the most complete of 

 the extinct, Paridigitata. Though such typical foot may be supposed 

 to have been pentadactyle, still, as not a single living or fossil 

 form has ever shown a trace or a rudiment * of the first digit (still 

 less this first digit in a developed state), I thought it more con- 

 venient to adhere to facts, and give the foot as it is found in the 

 most complete types, the first digit being always lost, and its carpal 

 and tarsal bone helping to support the second digit. This fun- 

 damental typical structure of the manus and pes may be stated, 

 in a few words, to be as follows : — 



►Supposing the foot to be pentadactyle, the two outer digits (the 

 fourth and fifth) are always supported in the manus and pes by one 

 single bone— the unciform in the manus, the cuboid in the pes ; the 

 three succeeding inner digits are supported each by a separate bone 

 — the third, second, and first cuneiform in the pes, and the os 

 magnum, trapezoideum, and trapezium in the manus. Besides, in 

 the manus, the third digit, being supported by the magnum, also 

 touches the unciform by a small ulnar projection, and the second, 

 supported by the trapezoides, goes to touch the os magnum ; the 

 second digit of the pes is supported by the second cuneiform, and by 

 its fibular projection is connected with the third cimeiform. The 

 first digit is lost in all Ungulata, and its t_\^)ical bone, the trapezium, 

 or first cuneiform, helps to support the second digit. 



* Prof. Huxley noticed tbis absence of rudiments of the first digit in bis An- 

 niversary Address of 1870 (Quart. Journ. Geol. Soc). Such rudiments of tlie 

 first digit, described in many cases, have proved always, on examination, to have 

 been mistaken, the trapezium or the first cuneiform being taken as the rudiment 

 of the first digit. 



