BRTTNIQTTEL, AND ITS ORGANIC CONTENTS. 
537 
sory lobe to, just as the indent g less definitely marks a corresponding production, o, 
of the lobe d. Thus, in the upper molar of the Horse, fig. 4, we define the ‘ antexternal 
lobe,’ a, the 4 postexternal lobe,’ b, the 4 antoblique lobe,’ c, the 4 postoblique lobe,’ d, and 
the 4 internal lobule,’ to ; also the islands, anterior, h , and, posterior, i, which are the 
dilated and severed terminations of the antoblique, e, and postoblique, g, valleys ; it be- 
comes necessary to note also the anterior indent, k, and the lobular production, o. 
The genus Ealoplotherium (fig. 2) manifests an instructive modification of the palaeo- 
therian upper molar surface approximative to that of Eguus. The indent, k, is present 
and so deep that the accessory lobule, to, continues longer disconnected from the ant- 
oblique lobe, c, than in Eguus ; both the oblique valleys, g and c, are also deeper. What 
renders this modification of the upper eocene Palseotherioid the more interesting and 
suggestive is, that the Equine animal ( Hipparion , fig. 3), of the succeeding (miocene) ter- 
tiary period, also long retains the accessory lobule, to, with its circle of enamel distinct. 
Lower molars. — Save in Proboscidians and Tapiroids the lower grinders of Perisso- 
dactyles are divided into five lobes (fig. 5, and Plates LVII.-LIX.) — two outer, a and b, 
and three inner, c, d, e — by one external, i, and two internal enamel-folds or indents, 
k and g. 
In Palaeotherium the internal folds are simple and wide, reducing the inner lobes to 
the mere internal terminations of the two outer lobes which constitute the main part of 
the grinding-surface. 
In Rhinoceros (fig. 5) the antinternal fold, k, resembles that in Ealaeotherium ; but the 
Eig. 5. 
Lower molar, Rhinoceros. 
Eig. 6. 
Lower molar, Equus. 
postinternal fold, g, slightly expands as it penetrates the crown, and better defines the 
postinternal lobe, e : it is continuous with the outer fold, i, until the crown is partly 
worn down ; prior to which there is an approach to the tapiroid or dilophodont type of 
grinding-surface. 
In Hipparion and Eguus (fig. 6 ) besides the antinternal fold, k, there is a midinternal 
fold, f, which is wide and shallow ; the postinternal one, < 7 , is narrow, deep, and expands 
abruptly at its termination, like k, in the fore-and-aft direction, after the pattern of the 
capital T. The antinternal fold, k, penetrating the fore part of the crown, dilates and 
runs along the same antero-posterior line as the terminal expansion of g, and divides the 
antinternal, c, from the antexternal lobe, a, just as the expansion of the fold, g, divides the 
lobes d and e from the lobe b. Finally, Hipparion shows short indents near the outer 
