19G CRITIQUES AND ADDRESSES. [ix. 



slender, and its distal end is ankylosed with the tibia. 

 There are three toes on the hind foot having similar pro- 

 portions to those on the fore foot. The principal meta- 

 carpal and metatarsal bones are flatter than they are in 

 any of the Equidce ; and the metacarpal bones are longer 

 than the metatarsals, as in the Palceotlieria. 



In its general form, Plagiolophus resembles a very 

 small and slender horse, 1 and is totally unlike the 

 reluctant, pig-like creature depicted in Cuvier's resto- 

 ration of his Palceotlierium minus in the " Ossemens 

 Fossiles." 



It would be hazardous to say that Plagioloplius is the 

 exact radical form of the Equine quadrupeds ; but I do 

 not think there can be any reasonable doubt that the 

 latter animals have resulted from the modification of 

 some quadruped similar to Plagiolophus. 



We have thus arrived at the Middle Eocene formation, 

 and yet have traced back the Horses only to a three-toed 

 stock ; but these three- toed forms, no less than the Equine 

 quadrupeds themselves, present rudiments of the two 

 other toes which appertain to what I have termed the 

 " average" quadruped. If the expectation raised by the 

 splints of the Horses that, in some ancestor of the Horses, 

 these splints would be found to be complete digits, has 

 been verified, we are furnished with very strong reasons 

 for looking for a no less complete verification of the 

 expectation that the three-toed Plagiolophus-like "avus" 

 of the horse must have had a five-toed " atavus " at some 

 earlier period. 



No such five-toed " atavus," however, has yet made its 

 appearance among the few middle and older Eocene 

 Mammalia which are known. 



1 Such, at least, is the conclusion suggested by the proportions of the skeleton 

 figured by Cuvier and De Blainville ; but perhaps something between a Horse 

 and an Agouti would be nearest the mark. 



