568 
PROFESSOR OWEN ON FOSSIL REMAINS OF EQUINES 
Fie. 3. 
Upper molar, 
Hipparion. 
fig. 1) rather more resemble, than do the Horse’s (Plate LVII. fig. 1), the present fossils 
in this character. The islands of dentine and cement (Plate LXII. figs. 1,4 , h, i) in the 
worn teeth are relatively larger to the rest of the grinding-surface in Equus arcidens. 
They are bounded toward the inner side of the tooth by a ridge of enamel more equably 
curved and more parallel with the outer ridge than in the common Horse or other old- 
world species of Equus : this ridge, however, is occasionally subject to secondary plica- 
tions, as shown in the section of the premolar, fig. 4, Plate LXII. 
The lobes c, d, in E. arcidens (Plate LXII. figs. 4, 5) are relatively 
narrower and more regularly crescentic. The lobe c sends off the 
peninsula of enamel m , which with the enclosed dentine is, in pro- 
portion to the crown of the tooth, much smaller and more simple 
than in all other known Equines, recent or extinct, excepting the 
Brazilian E. neogceus and E. principalis , Ld. (figs. 9 & 10) ; it is not, 
however, an insular summit of a distinct column of enamelled den- 
tine as in the Miocene Hipparions ( Hippotherium , Kaup) (Cut, 
fig. 3, m). 
In both the worn molars of Equus arcidens the peninsular lobule m first extends 
inward and then curves backward, the enamel-wall very slightly expanding in the 
backward curve. The end of the entering peninsula of cement, e, is more expanded 
as the projecting peninsula, m, is more contracted: in m 2, fig. 5, it is not encroached 
upon by any enamel-fold from the lobe c ; but it is so in the teeth p 3 and 4. The fore 
part of the peninsula m extends forward more or less hi all other newer tertiary Equines, 
save E. neogceus and E. principalis, as it does in all existing species (as at p, Plates LVII.- 
LX.) ; but less so in the Ass- and Zebra-groups, in which the process m is relatively 
smaller than in the Horse. Thus the more simple shape as well as smaller size of the 
process m in Plate LXII. figs. 1, 4, 5, 9, 10, is amongst the well-marked character- 
istics of the upper molars of the group represented by Equus arcidens, E. neogceus, and 
E. principalis . 
The entering fold, g, figs. 1, 4, 5, Plate LXII., which marks off an appendage of the 
lobe d, is relatively deeper in Equus arcidens than in E. caballus ; and the process, o, 
of d is more distinct and more like a posterior serial repetition of the process m in 
E. arcidens. 
The lines of enamel, generally, on the grinding-surface of the upper molars of Equus 
arcidens, are less interrupted by folds and waves than in most Equidce, showing in this 
respect the opposite extreme to those in the extinct Equus plicidens, Ow.*, of European 
postpliocene beds. But the most conspicuous and readily appreciable character of the 
present teeth from the postpliocene (1) beds of Paysandi is the greater degree of lon- 
gitudinal curvature of the entire tooth (Plate LXII. figs. 3, 7), the molars retaining 
the generic Equine character in the length of the crown before it becomes divided into 
roots. 
* History of British Fossil Mammals, 8vo, 1846, p. 393, fig. 153. 
