EROM CENTRAL AND SOUTH AMERICA. 
567 
pi. vii. of Castelnau’s ‘ Expedition dans les Parties centrales de l’Amerique du Sud,’ 
&c., 4to, 1855. The four molars from the Arroyo Gutierrez (Plate LXII. figs. 1-8) 
show the same mineral characters and coloration as do the bones and teeth of the Glyp- 
todon and Megatherioids from that locality, and beget a conviction of their being fossils 
of the same age. 
The molars are from the right side of the upper jaw, and probably belonged to the 
same head. Two show attrition of the grinding-surface ; and two, with the body of the 
tooth fully formed, had not cut the gum. 
The larger of these unused grinders is probably^? 4 (I made a section of this tooth, 
which is figured in Plate LXII. fig. 4) ; the smaller is m 8 (ib. fig. 8). The larger of the 
worn teeth may be^? 3 (ib. figs. 1, 2, 3), the smaller one is m 2 (ib. figs. 5, 6, 7). 
There is strong analogical probability that, in the present as in other species of Equines, 
the premolars exceeded the true molars in size (as, according to the above determina- 
tions, they do) in the degree shown in figures 1, 4, 5, 8. 
The proportional smallness of the m 3 (ib. fig. 8) resembles that in the similarly 
unworn tooth of the Zebra {ante, Plate LX. fig. 1, m3) and Equus spelceus, Ow. 
(ib. Plate LX. fig. 4, m 3). In all these the disparity is less marked when the tooth 
m 3 comes to be worn down to its thicker part ; it would then, in Equus arcidens , 
exceed, in regard to relative size to the other teeth, the last molar in Equus conversidens 
(Plate LXI. fig. 1, m 3). 
Comparing the tooth which I regard as p 4 (Plate LXII. fig. 4) with that in the 
Equus fossilis from the Oreston Cavern (Cut, fig. 1), and comparing p 3 of E. arcidens 
with p 3 of Equus plicidens (Cut, fig. 2), or comparing both teeth of E. arcidens with 
Eig. 1. 
Upper molar, p 4, Equus fossilis. Upper molar, p 3, Equus plicidens. 
their homologues in the Horse'(i?. caballus , Plate LVII., fig. 1, p 3, p 4), the antero- 
external groove f is deeper and more semicylindrical in E. arcidens ; the dividing ridge, 
ib. n, is narrower, more produced, not pinched in at the base as sometimes ivcE. caballus , 
nor grooved along the summit (Plate LXII. fig. 1, p 3, n). The dentinal lobes in E. arci- 
dens (Plate LXII. fig. 1, a, b) are narrower transversely, or from the outer to the inner 
side of the tooth, than in the Horse, and, from the more equal and regular curve of the 
inner enamel-wall of the lobes, which is nearly parallel with the outer one, these lobes are 
more definitely crescentic : the Ass’s (Plate LVIII. fig. 1) and Zebra’s molars (Plate LX. 
4 g 2 
Eig. 2. 
