1873.] 



Osteology of the Hyopotamidae. 



157 



We must briefly consider each of these groups. 



The Paridigitata with crescentic teeth following the inadaptive mode of 

 reduction, and whose skeletons are known, are the Anoplotherium, Xipho- 

 don, Anthracotheridce, and Hyopotamidce. If it should be asked why they fol- 

 lowed this mode of reduction, the reason is obvious. Admitting that an 

 advantage is gained by the simplification of the foot and the reduction of 

 the number of digits, this mode of reduction is the most simple course to 

 be taken. We must imagine the enlargement of the middle digits to be 

 accompanied by a broadening of their correspondent bones in the carpus 

 and tarsus ; the trapezoideum and the second cuneiform were simply 

 pushed aside (not made use of) by the enlargement of the third digit, and 

 their reduction kept pace with the reduction of the second digit. If we 

 think how the process must have gone on " in natura" we shall find that 

 it required quite an unusual occurrence, some happy chance, for the third 

 digit to go over the separating line between the magnum and trapezoideum, 

 or the third and second cuneiform, and get a footing on these last bones, 

 which typically belonged to the second digits. This was evidently the most 

 advantageous mode, but it did not occur at once ; and the organism has 

 taken the more simple and obvious inadaptive mode, which, once fairly set 

 in, could not be changed. This branch of the Paridigitata then, starting 

 from their tetra- (or penta-)dactyle progenitors in the Cretaceous or earliest 

 Eocene, arrived at the close of the Eocene (from which strata alone we have 

 Paridigitata whose skeletons are known) to the reduced didactyle forms, 

 known as the Anoplotherivm and Xiphodon. That these last had tetra- 

 dactyle ancestors is supposed, on theoretical grounds, by the evolutionists ; 

 besides, their rudimental second and fifth digits point clearly to some 

 form in which these rudiments were completely developed and used for 

 locomotion. 



Whilst trying to ascertain the structure of the skeleton of an extinct 

 family {Hyopotamidce) allied to the Anoplotheridce, but which was sup- 

 posed to be chiefly Miocene, I found that the Miocene genera could be 

 regarded only as the last representatives of this exceedingly numerous 

 family, whose chief development fell in the Eocene times, when it was 

 represented by numerous subgeneric and even generic forms. I was 

 fortunate enough to find, ni the collection of M. Aymard, at Puy, a 

 large assemblage of bones belonging to the oldest Miocene representative 

 of this family, the Hyopotamus ; indeed so much, that I could completely 

 restore the limbs and nearly the whole skeleton. The limbs prove to be 

 tetradactyle, with well-developed lateral digits. The same family is so 

 richly developed in the Eocene, that we have a full right to suppose that 

 the older genera had even a more completely developed manus and pes. 



Erom Puy I came to London to complete my study, as teeth which were 

 not to be distinguished from the Hyopotamus of Puy were known to be 

 numerous in England ; and whilst studying the bones found in England, 

 I was struck by the fact that some of these belonged to a didactyle genus 



