1873.] 



Osteology of the Hyopotamidse. 



161 



stage I must consider a small Suilline animal, though not the oldest, but 

 perhaps a remnant of the older type ; this is the Choerotlierium, Lart., 

 from Sansans. The primitiveness of this small pig is indicated by the 

 fact that the carpal and tarsal bones retain their typical relation to the 

 four metacarpals and metatarsals ; the humerus is very Anoplotherium-like, 

 and the distal extremity of the metapodium is smooth anteriorly, the 

 phalangeal articular ridge being limited only to the palmar side, as in all 

 ancient Paridigitata. 



First Stage, Choerotlierium, Lart., Sansans. — The middle digits are en- 

 larged, but the laterals still retain their typical relation to the supporting 

 bones of the carpus and tarsus*. Distal end of humerus Anoplotherium- 

 like (ancient) ; the proximal end of the radius, in correspondence with the 

 humerus, is also Anoplotherium-\ike. The distal end of metapodium is 

 smooth, the phalangeal ridge being limited to the palmar side. 



Second Stage, Paloeochoerus (Allier). — The adaptive reduction of the 

 manus and pes has fairly set in, its first indication being that the radial 

 margin of the third digit (in the manus and pes) is raised in such a way 

 as to exclude the second digit from going to its typical facet on the os 

 magnum and third cuneiform, though leaving it still in the full pos- 

 session of the trapezoid and second cuneiform. The phalangeal arti- 

 cular ridge is passing from the palmar side round the distal extremity to 

 the anterior face of the metapodium. 



Third Stage, Suidce. — Adaptive reduction is proceeding further, the 

 middle digits are greatly enlarged, and the third digits of the manus and 

 pes spread over one half the trapezoideum and nearly the whole of the 

 second cuneiform. The lateral digits touch the ground only very slightly, 

 and are not important for locomotion. 



Fourth Stage, Dicotyles. — The middle digits are so enlarged and adapted 

 that the entire distal surface of the carpus and tarsus is taken by them ; 

 the lateral digits have no distinct facets on the distal surface of the 

 carpus, and are merely hanging to the enlarged middle digits. The fifth 

 digit of the pes is lost, and the two metatarsals are coalesced into a 

 cannonbone; the metacarpals are also so closely pressed together that 

 their confluence is imminent. The complication of the stomach, which is 

 divided into three chambers, shows a beginning of rumination, slight traces 

 of which are even exhibited by the common hog ; the premolars become 

 complicated, and begin to assume the shape of molars t, the first premolar 

 is lost (as in all Ruminants), the incisors reduced to four, the canines 

 are small. 



Fifth Stage. — The culminating point is not yet reached by the tubercular- 



* That is, the second digit is supported by the trapezoideum, and has besides a facet 

 on the os magnum, as in Hippopotamus, or in the typical tetradactyle foot generally. 



f A very important circumstance, considering that we meet the same fact in other 

 groups where the premolars assume the shape of molars, as in Palmotheridce, horses 

 rhinoceros, &c. 



