178 Royal Society : — 



metacarpals and metatarsals ; the humerus is very Anophtherium- 

 like ; aud the distal extremity of the metapodium is smooth an- 

 teriorly, the phalangeal articular ridge being limited only to the 

 palmar side, as in all ancient Paridigitata. 



First Stage, Choeroiheriwm.'L&Tk. (Sansans). — The middle digits are 

 enlarged, but the laterals still retain their typical relation to the sup- 

 porting bones of the carpus and tarsus *. Distal end of humerus 

 Anojilotherium-like (ancient) ; the proximal end of the radius, in 

 correspondence with the humerus, is also Anoplotherium-like. The 

 distal end of metapodium is smooth, the phalangeal ridge being 

 limited to the palmar side. 



Second Stage, Palceochcerus (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 goiug to its 

 typical facet on the os magnum and third cuneiform, though leav- 

 ing it still in the full possession of the trapezoid and second cunei- 

 form. The phalangeal articular ridge is passing from the palmar 

 side round the distal extremity to the anterior face of the meta- 

 podium. 



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 meta- 

 tarsals 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 f, 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-toothed Paridigitata following the adaptive mode of re- 

 duction ; but as it was reached by the same group on the inadaptive 

 mode (Entelodon), and as the parallel group of crescent-toothed 

 Paridigitata, whose reduction is going at a quicker rate, has already 

 reached it, there can be no doubt that the Suina are tending also 



* 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 with the same fact 

 in other groups where the premolars assume the shape of molars, as in Palceo- 

 fkcridep, horses, rhinoceros, &c. 



