FROM CENTRAL AND SOUTH AMERICA. 
565 
probable that those of the deciduous molars of the Equus fossilis from Kent’s-Hole 
cavern may have resembled in their proportions those of the extinct Mexican Colt or 
Filly, represented in Plate LXI. fig. 3. I append, in millimetres and their English 
equivalents, the 
Dimensions of the grinding-surface of the upper molars of Equus conversidens. 
P 2- 
Antero-posterior breadth* . 
metre. 
. 0-030 
_ 
inch. 
1 
lines. 
91 
^2 
Transverse breadth* 
. 0-025 
= 
1 
0 
p 3. 
Antero-posterior breadth 
. 0-025 
= 
1 
0 
Transverse breadth 
. 0-025 
= 
1 
0 
m 1. 
Antero-posterior breadth . . . , 
. 0-022 
----- 
0 
m 
Transverse breadth 
. 0-021 
= 
0 
10 
m 3. 
Antero-posterior breadth . . . 
. 0-019 
= 
0 
9 
Transverse breadth 
0-018 
= 
0 
rH|<M 
00 
Equus tau, Ow. — This species is indicated by a series of five grinders of the upper 
jaw, Plate LXI. fig. 4, and of three grinders of the lower jaw, ib. fig. 5. The upper 
grinders include the three true molars ( m 1-3) and the two contiguous premolars (p 3, 
p 4). They are as much smaller than the corresponding teeth of Equus conversidens as 
are those of the Asinus fossilis from the Oreston Cavern (Hist, of British Fossil Mam- 
mals, p. 396, figs. 157, 158) compared with the teeth of Equus plicidens (op. cit. p. 392, 
figs. 152, 153) from the same cavern, and they indicate a species about the size of the 
common Ass. 
As compared with any of the smaller existing kinds of Equines (Plates LVIIL, LIX.), 
the antero-posterior diameter of the grinding-surface of the crown, especially in the 
premolars, is in excess ; and in this character Equus tau also differs from Equus con- 
versidens, as it does in the greater relative size, especially antero-posterior breadth, of 
the last molar, fig. 4, m 3 : E. tau further differs in the greater flattening, from without 
inward, of the inner lobe, m , of most of the molars. 
The lower molars of Equus tau (Plate LXI. fig. 5, p 2, 3, 4) conform to their 
homotypes above, in their character of narrowness from without inward ; the postero- 
internal enamel-fold, g , describes more neatly or definitely than that in other Equines 
the figure of a short-stemmed capital letter T, which suggested the nomen triviale of the 
second extinct Mexican species. 
The fossils representing both species were discovered by Don Antonio del Castillo in 
the posttertiary deposits, above mentioned, of the Valley of Mexico. 
Equus arcidens, Ow. — I have already alluded to the interest with which one views 
the direction of the deviations of the extinct American Equines from the ordinary type 
in connexion with the curvature of the grinders, the length of the crown, and the ante- 
riorly converging curve of their sockets, as exemplified in Equus curvidens and E. con- 
* Across middle of crown. 
4 G 
MDCCCLXIX. 
