264 ON THE EXTINCT MAMMALIA OF 



noticed, being true fossils and belonging to the deposits in which they were dis- 

 covered. Many are undistinguishable in anatomical character from corresponding 

 bones and teeth of the recent Horse, and some of them perhaps have really belonged 

 to the latter, and are not true fossils, but accidental occupants of the formation in 

 which they were found ; that is to say, they may have been buried in the formation 

 long subsequent to its deposit, and have thence derived certain of the characters, 

 such as coloring and petrifaction, of the true fossils of the formation. Other teeth, 

 usually of comparatively large size, exhibit a degree of complexity in the plication of 

 the enamel upon the triturating surface that they are readily distinguished by this 

 character from those of the recent Horse. 



The two varieties of teeth indicated are related to each other much in the same 

 manner as are those of Equus fossilis and Equus plicidens of Europe, and the palaeon- 

 tologist who is disposed to place no value upon a wide separation of locality in the 

 attempt to determine a species, might consider the two varieties of equine fossils of 

 the United States the same as the European ones just named. 



It is true that between the usually smaller and more simple, and the larger and 

 more complex varieties of fossil equine teeth of the United States, there are interme- 

 diate forms, which entirely preclude a well-defined grouping of the specimens so as to 

 represent two distinct species. 



I nevertheless suspect that the fossils represent two different species, according to 

 the ordinary comprehension of a species among living equine animals, and with this 

 view I have distinguished them by the names of Equus fraternus and E. complicatus. 

 The latter name, substituted for that of E. amev'kanus, is most probably synonjnnous 

 with the E. major of Dr. Dekay, who, as previously mentioned, in alluding to the 

 discovery of fossil teeth of a Horse in America, remarks that they resemble those of 

 the common Horse, but from their size apparently belonged to a larger animal. 



The reference of the fossil remains to two distinct species I think receives support 

 from the circumstance that if teeth of several existing members of the genus Equus 

 were commingled they would be even less readily distinguishable from one another. 



The existing species of Equus are mainly distinguished by external characters, 

 such as coloration and ornamentation of the hair, and variation in the size of the 

 animals or their parts. If bones and teeth of the Domestic Horse, the Mule, the 

 Ass, the Dziggetai, the Hemione, the Quagga, the Dauw and the Zebra were com- 

 mingled, they might readily be considered as belonging to varieties of a single species. 

 Though I have no opportunity of making the comparison, I suspect that the bones 

 and teeth of the three last-named species are so nearly alike that, had they been 

 found in a fossil state in Southern Africa, instead of the living animals, they would 

 have been unhesitatingly considered as pertaining to a single species. 



