176 Royal Society : — 



librium. Now the confluence of the two middle digits is always 

 followed by a considerable contraction ; and if this coalescence 

 should occur in the imperfectly adapted foot of Anoplotherium, and 

 especially Xvphodon, all equilibrium would be lost. If ever such 

 confluence occurred, by reason of the tendency to the greatest pos- 

 sible reduction, the resulting form had not the least chance of being 

 propagated and of holding its ground against the competing genera. 

 The broadening of the middle digits could not occur after the entire 

 loss of the laterals ; and we shall see that, in genera which have left 

 immediate successors (Sus, Hyomosclws), the lateral digits are not 

 allowed to go until the middle ones have obtained a secure footing 

 on the entire distal surface of the carpus and tarsus. However, 

 these inadaptively reduced genera of the Eocene could perhaps have 

 lived till our own days ; but the development of the competing and 

 better adapted forms pressing them on all sides, they had no chance 

 to stand their ground against them, and became extinct without 

 any direct posterity, while the succession of the Paridigitata Sele- 

 nodonta was carried by a side branch, and reached its culminating 

 point in the Miocene, continuing from then to our own days. 



We turn now to the same mode of inadaptive reduction as mani- 

 fested by the tubercular-toothed Paridigitata (Bunodonta), or Suina. 

 The old representatives of this group are very little known. The 

 Choeropotamus is a very doubtful genus, and may be inclining towards 

 the crescentic-toothed Paridigitata, being supposed to be the pro- 

 genitor of the Anthracotheridce and Hyopotamidce. Besides it we have 

 the Acotlierulum saturninum, Ger., a truly tubercular-toothed Paridi- 

 gitate from the Upper Eocene, Acotlierulum Campicliii (Dichobune 

 Camp., Pictet) from the Loiver Eocene of Mauremont, and a larger 

 pig-like animal from the same deposit not yet described or named. 

 These are undoubtedly the oldest tubercular-toothed Paridigitates 

 we know ; but unfortunately our knowledge is based only on dental 

 characters. However, considering that even the recent Suina have 

 not yet completely lost their two lateral digits, it may, with the 

 greatest probability, be inferred that these old Eocene forms were 

 tetradactyle. Our knowledge of the development of this group is 

 very incomplete ; but there can be no doubt that, though not nearly 

 so rich as the Selenodont group, they were still numerous, as may 

 be inferred from the great quantity of the Suina in the Miocene, 

 and such forms as the Listriodon splendens *. We are so accus- 

 tomed to look on the Suina as a group of tubercular-toothed tetra- 

 dactyle Paridigitata, that no one ever thought of the possibility of a 

 didactyle hog ; but, strange as it may seem, such a Suilline animal 

 existed ; stranger still, it existed in such an ancient period as the 

 close of the Eocene in the lowest strata of Eonzon at Puy. This is 

 the Entelodon, Aym. (Elotherium, Pom., Archceotherium, Leidy). 

 The Suilline characters are so striking in this form, that it was at 



* I have not been so fortunate as to see any bones of the Listriodon ; but 

 as this miocene hog died without any successors, I should not be astonished if 

 it prove to be didactyle, thus being a parallel to Hyopotamns in the same 

 sense as Entelodon is parallel to Anoplotherium. 



