1 74 Royal Society : — 



mode of reduction, and whose skeletons are known, are the Anoplo- 

 therium, Xiphodon, Anthracotheridce, and Hyopotamidce. If it should 

 be asked why they followed this mode of reduction, the reason is 

 obvious. Admitting that an advantage is gained by the simplifica- 

 tion 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 ima- 

 gine 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 re- 

 duced didactyle forms, known as the Anoplotheriwm and Xiphodon. 

 That these last had tetradactyle 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 AnoplotJieridce, but which 

 was supposed 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, in 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 tetra- 

 dactyle, 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. 



Prom 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 of the same family, which in England 

 proved to be associated with the tetradactyle genus. To this new 

 genus of the Hyopotamoid family I gave the name Diphpus. This 



