Sept. 1847.] 265 



curvidens as described by Mr. Owen, more than with any other species, so far 

 as I am capable of judging, which ha9 made me refer them to that species. 



The second species is founded upon twelve specimens of teeth which have 

 been deposited in the cabinet of the Academy by our enterprising fellow-mem- 

 ber, Dr. M. W. Dickeson, and is one only of the many important results of his 

 palaeontological researches in the southwestern part of the United States. 

 Ten of these interesting relics, consisting of five superior and five inferior mo- 

 lars, Dr. Dickeson states * were obtained, together with remains of the 

 Negalonyx, Ursus, the os hominis innominatum fossile, &c, in the vicinity of 

 Natchez, Mississippi, from a stratum of tenacious blue clay underlying a diluvial 

 deposit. The remaining two, both right superior posterior molars, are rolled 

 or water-worn, and were found, as Dr. D. informs me. upon one of the Natchez 

 Islands, in the Mississippi River. All the specimens have the exterior 

 cementum entirely removed, with the exception of one inferior molar of the 

 right side, in which it still exists upon the external face, and much of the in- 

 ferior cementum, and part even of the dentine, is also destroyed, so that the 

 enamel folds everywhere stand out in strong relief. 



These teeth are larger than those of any species heretofore known, recent 

 or fossil, and must have belonged to a horse, which, in point of magnitude, 

 was a fit cotemporary for the Mastodon, Elvphas, $c. , and worthy of the large 

 continent which produced it, and I have therefore named it Equus Ameri- 

 canus. 



Two of the inferior molar teeth measure 4.3 inches in length, with a lateral 

 diameter of 1.25 of an inch, and a transverse diameter of .7 of an inch. Two 

 also, of the superior molars, measure 3.9 inches in length externally with a 

 lateral diameter of 1.2 of an inch, and a transverse diameter of 1.1 of an inch. 

 The inferior molars are curved from without inwards, instead of laterally, as is 

 usual. The superior molars are curved to a degree intermediate to that of the 

 Equus caballus and Equus curvidens. 



The enamel folds are one-fourth thicker than in the recent horse and the 

 isolated enamel folds of the superior molars are much more plicated, resem- 

 bling in this respect the Equus plicidens, Owen. In one of the two superior 

 posterior molars, there is an additional or third isolated enamel fold, which 

 is oval, and two or three times larger than in the recent horse, and in the 

 other there is a fourth, small, round, isolated enamel fold. Both of these 

 teeth indicate a greater amount of room for development, and consequently a 

 larger jaw. Other and considerable differences will be noticed upon com- 

 paring the figures 2 and 3, representing the crowns of these teeth, especially 

 at the posterior part, which might lead to the supposition that they belonged 

 to distinct species, but from the general characters of the two specimens, 

 added to reasons before stated, relative to the amount of variation existing in 

 the corresponding tooth of the recent horse, I cannot but think they both be- 

 long to Equus Americanus. 



*Proc. Acad. Nat. Sci., vol. 3, p. 106. 



