FROM CENTRAL AND SOUTH AMERICA. 
563 
proceed to the evidence of extinct Equines, more nearly allied to these latter, from a 
locality in Central America. 
I Avas favoured, through the intervention of It. T. C. Middleton, Esq., of the British 
Legation, Mexico, in the year 1866, by receiving from Don Antonio del Castillo, 
Engineer of Mines, resident in Mexico, a series of specimens, casts, and photographs of 
objects of Natural History, including some fossil remains discovered by the liberal donor 
in newer Tertiary or Quaternary deposits in the valley of Mexico. 
In this series were evidence of at least two species of Equus, consisting of portions of 
jaws with teeth, and detached teeth, found fossil associated with remains of Mastodon 
and Elephas and of an extinct Cervus. 
One of the Equine species ( Equus conversidens, Ow., Plate LXI. fig. 1) so far corre- 
sponds in the size, curvature, and pattern of the grinding-surface of the upper molar teeth 
with Equus curvidens, Ow. (ib. fig. 2) as would have led me to refer it thereto. But 
the chief Mexican fossil yields a character incompatible with an indication of the 
arrangement of the teeth in Equus curvidens, and which I have not before met with in 
any kind of horse, viz. a curved convergence of the two series of upper grinders towards 
the fore part of the palate to a degree exceeding that in other Equines (compare fig. 1 
with fig. 6, Plate LXI.) ; and this peculiarity, with the curvature of the molars them- 
selves, suggests a most interesting and significant resemblance to characters of the 
upper molars in Toxodon and Nesodon*. 
I therefore regard this Mexican fossil as representing a species which I propose to 
call Equus conversidens f. 
The specimen (Plate LXI. fig. 1) is a portion of the upper jaw with the right and left 
series of grinders, and a considerable part of the intervening bony palate. Each dental 
series includes, as in the rest of the Equine family, three molars and three premolars. 
The premolars exceed the true molars in size to a greater degree than in Equus asinus 
(Plate LVIII. fig. 1) : the true molars, m 1, 2, 3, in Equus conversidens , e. g., equal p 4, 
p 3, and the posterior lobe of p 2, while in E. asinus they equal p 4, p 3, and two- 
thirds of p 2, in longitudinal extent of grinding-surface. The disproportion between 
this extent of the true molars and that of the premolars is still greater in E. conver- 
sidens as compared with E. caballus (Plate LVII. fig. 1). 
The last molar, m 3, fig. 1, Plate LXI., is relatively smaller than in any old-world. 
Equine. The first premolar, p 2, resembles in the minor production of the anterior 
lobe that tooth in Equus asinus, Equus quagga, and differs in this respect from E. 
caballus. The grinding-surface, however, retains, as in Equus aff. caballo, Ld., and in 
E. curvidens, the general conformity of character of enamel-folding so remarkable in all 
the modern and in the European postpliocene Equine species hitherto described. 
As compared with Equus caballus (Plate LVII. fig. 1), the dividing ridge (Plate LXI. 
* Compare Plate LXI. fig. 1 with pi. xv. fig. 3, Philosophical Transactions, 1853, and with pi. 1, ‘ Eossil 
Mammalia of the Beagle,’ 4to, 1840. 
f Prom converto, to turn towards ; or conversus, turned towards, and dens , a tooth. 
