152 



Dr. W. Kowalevsky on the 



[Feb. 6, 



crescentic, so thick are the lobes ; but once this uncertain stage is passed, 

 both groups keep unmistakably distinct. 



Having once become convinced that these two groups of crescent-toothed 

 and tubercular-toothed Paridigitata, after branching off from a common 

 progenitor in the early Eocene (perhaps the Cretaceous) period, followed 

 diverging lines of descent, never mixing together, I tried to ascertain ac- 

 curately, by such data as were furnished by fossil remains and by lawful 

 induction, what are the exact modifications of the skeleton exhibited by 

 each group along the ascending and descending lines. As these modifica- 

 tions 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 tendency 

 clearly manifested by them to a gradual reduction of the manus and pes 

 in such a way that each descendant is always somewhat more reduced 

 than its immediate predecessor. The limbs 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 

 lines, 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 assem- 

 blage 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 Anthraco- 

 therium, 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 

 convenient 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 fundamental 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 



* Prof. Huxley noticed this absence of rudiments of the first digit in his Anniversary 

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



