FROM CENTRAL AND SOUTH AMERICA. 
569 
Owing to the non- production of the fore part of the lobe m (figs. 1, 4 & 5, Plate LX1I.) 
the groove r (ib. ib.) is less marked, and appears on the inner concave surface (fig. 2) 
rather than on the fore and flattened surface of the tooth as in E. caballus and E. cur- 
videns (fig. 16). 
The longitudinal groove, e, on the concave inner surface (fig. 2), at the entry of the 
enamel-fold (fig. 1, e), is nearer the middle of that surface, as in Equus principalis, Ld., 
and is less near the posterior angle than in other Equines; and there is no groove 
answering to the middle of the inner surface of the process m, which process is there 
more or less indented in most other Equines. On the outer or convex surface (Plate 
LXII. fig. 6) the simplicity, narrowness, almost sharpness of the produced mid ridge, h, 
adds a differential character to the great longitudinal convexity of that surface in Equus 
arcidens. The posterior surface (fig. 3) is slightly concave from within outwards, shows 
a feeble indication of the entry of the fold g, and, owing to the minor production of the 
external angle, a greater proportion of the posterior longitudinal channel, f, is visible, 
looking at the back part of the tooth, than in other Equines. 
The upper molar tooth representing the Equus neogceus of Lund (op. cit., IV. tab. xlix. 
fig. 3) indicates a species as much smaller than Equus arcidens as is the Ass than the 
Horse. It is determined by that acute and careful observer as being the fourth molar of 
the right upper jaw (Fjerde Kindtand i ho'ire Overkjeeve, tom. cit. p. 93) ; it answers very 
nearly to the true molar which, in Equus arcidens, is represented in fig. 5, Plate LXII. 
By reference to the copy of Lund’s figure in fig. 9 of this Plate, it appears that the 
lobe c extends, in E. neogceus, nearer to the antinternal angle, and communicates, through 
attrition, with the dentine of lobe d : the posterior surface of the tooth is, likewise, rela- 
tively narrower, the lobule o smaller, and the entering fold g shallower. These differ- 
ences, with that of size, and in the absence of any knowledge of the degree of general 
curvature of the entire molar, forbid a reference of Equus arcidens to E. neogceus. 
The Equus principalis, Ld., is represented by an upper molar indicative of a horse 
of equal size with Equus arcidens. The fossil molar tooth from the older breccia of the 
Brazilian cavern, figured in tab. xlix. fig. i. tom. cit. (copied in fig. 10, Plate LXII.), is 
determined by Lund to be the fifth molar of the right upper jaw, =m 2 (“ Eemte Kind- 
tand i hoire Overkjaeve,” p. 93). It cannot be either the first, p 2, or the last, m 3, of 
the series ; yet it differs more from the ordinary shape of the grinding-surface of the 
Equine molars, well preserved in E. arcidens, than do either of those modified molars 
at the two extremes of the series. The molar representing Lund’s Equus principalis 
offers an interesting and suggestive resemblance to the upper molars of Palceotherium 
and to some teeth of species of Rhinoceros, in the contraction of the crown toward the 
inner surface, whereby the ordinary subquadrate form of the section of the upper Equine 
molar (as shown in all the figures illustrative of the present and preceding Papers) is 
exchanged for a subtriangular form, with the inner angle obtusely rounded off. Never- 
theless all the characters indicated by letters in the figures above-cited, especially fig. 9, 
are present in E. principalis, Ld. The terminal lobules, to, o, of the lobes c, d are equal 
