268 Prof. Owen on Fossil Teeth of Equines. [Feb. 4, 



the grinder, with a certain resemblance of enamel-pattern to the grinding- 

 surface of the E. curvidens, they show a greater degree of curvature of the 

 alveolar series of the upper jaw, with corresponding greater convergence of 

 the right and left molar series toward the fore part of the palate, than in 

 any previously described species of Equus. 



Deciduous teeth of the Equus conversidens from the same deposits of the 

 Valley of Mexico are described. Having determined these corroborative 

 and distinctive characters of aboriginal and now extinct American horses, 

 the author remarks, "It is unlikely, seeing the avidity -with which the 

 Indians of the Pampas have seized and subjugated the stray descendants 

 of the European horses introduced by the Spanish ' Conquistadors ' of 

 South America, and the able use the nomad natives make of the multitu- 

 dinous progeny of those war-horses at the present day, that any such 

 tameable Equine should have been killed off or extirpated by the ancestors 

 of the South-American aborigines." If, therefore, the fossil Equine teeth 

 do belong, as the author deems that he has proved, to a species distinct 

 from Equus caballus, Linn., " the circumstances of their discovery, and the 

 fact of the extinction of such (curvident and conversident) species of Horse 

 would point to some other cause than that of man's hostility to so useful an 

 animal, and such doubt as to extinction by human means may then be 

 extended to the contemporaries of the Equus curvidens and E. conversidens, 

 viz. Megatherium, Mylodon, Toxodon, Nesodon, Macrauchenia, Glyptodon, 

 Mastodon, &c." 



The author next proceeds to describe fossil teeth from the upper and 

 lower jaws, discovered by Don A. del Castillo in the same deposits of the 

 Valley of Mexico,, and referable to a third species of Equus, viz. Equus 

 tau, Ow. Finally the author proceeds to the description of some fossil 

 upper molar teeth from Pampas deposit, in the bed of a brook falling into 

 the " Arroyo Negro " near Paysandi, Monte Video, showing characters 

 more decisively distinct from any other known species of Equus than have 

 hitherto been described. 



The degree of curvature of the upper molar teeth exceeds that in Equus 

 curvidens, and equals that in Toxodon; and the specific name "arcidens" 

 is accordingly proposed for this aboriginal American species of Horse. 

 It is compared with so much of the characters as have been given by Dr. 

 Lund of his Equus neogceus and E '. principalis from Brazilian caverns ; and 

 the differences from all other Equines which these species and the E. 

 arcidens agree in presenting lead the author to view them as having, like 

 the Hippotherium of Kaup, formed a generic group in the Equidce, for 

 which he proposes the name Hippidion. 



The fossil teeth of H. arcidens were found associated with remains of 

 Megatherium and Glyptodon in the above-named locality ; the specimens 

 were transmitted and presented to the British Museum (in 1867) by the 

 Hon. W. G. Lettsom, Her Britannic Majesty's Minister at Monte Video. 



This paper is illustrated by drawings of the specimens described. 



