792 ANATOMY OF VERTEBRATES. 



Hants., by the Palceotherium aurelianense from the ' molasse 

 marine ' of Orleans, 1 and by the Palceotherium hippo'ides of the 

 lacustrine calcareous beds of Sansan, all which deposits are mi- 

 ocene, or are transitional between eocene and miocene. In the 

 first-cited example, the swollen termination of the lobe of the 

 molar, answering to c, m, fig. 268, remains longer as a detached 

 column, m, fig. 269. In the two other Palaeotherioids, the whole 

 foot is longer and more slender, with a longer and thicker middle 

 toe, than in the older eocene type -genus, whence the generic 

 name Anchitherium applied to them by von Meyer. 2 It is in- 

 teresting, also, to find that the transitional character is further 

 marked by the smaller relative size of first premolar, whereby 

 Anchitherium intervenes, as in the modification of the feet, be- 

 tween the Palceotherium and Hipparion. 



Thus amply and satisfactorily has been fulfilled Cuvier's 

 requisition of 1821 : * Entre le palasotherium et les especes 

 d'aujourd'hui Ton devrait decouvrir quelques formes interme- 

 diaires.' How, then, is the origin of these intermediate gradations 

 to be interpreted ? One may first remark, that as Palceotherium, 

 Paloplotherium, Anchitherium, Hipparion, and Equus, differ from 

 each other in a greater degree than do the Horse, Zebra, and 

 Ass, the difficulty of interbreeding would be greater, and the 

 probability of fertility less, supposing those extinct genera to 

 have co-existed. One cannot doubt, also, that every Avell-marked 

 species of these genera paired within itself, and that they exem- 

 plified respectively the character of a ' group of individuals de- 

 scended from common parents, or from such as resembled them 

 as closely as they resembled each other.' They did not, however, 

 exist as species, during the same periods of time, far less so 

 * from the beginning of things.' The single-hoofed Horse- 

 family cannot be traced further back than the pliocene tertiary 

 period : the tridactyle equine species have not been found in 

 strata earlier than miocene, and disappear in the upper eocene : 

 the heavier-bodied shorter-legged species with three functional 

 hoofs to each foot belong to upper and middle eocenes. Further- 

 more, in the oldest eocene (London clay, super-cretaceous Con- 

 glomerates and Plastic clay at Meudon, Paris), we get evidence 

 of Ungulates (Pliolophus, Hyracotherium, Coryphodori), in which 

 the perisso- and artio-dactyle characters were less differentiated 



the tipper eocene at Velay, e.g., ere Palceotherium proper had passed away. (Bulletin 

 du Congres Scientifique de France tenu a Puy, 1855.) 



1 Also in the upper eocene of the Basin of the G-aronne, with Acer 'other turn. 



2 Anchitherium occurs, also, in the ' marine molasse,' or lower miocene, of St. 

 Genies, Languedoc. 



