196 NORTH-CAROLINA GEOLOGICAL SURVEY. 



each case with races or species quite different from those 

 among whom it was first connected or who were its cotern- 

 poraries. 



The cause of the extinction of so many species, is a mystery. 

 The fact is well established, but it is only in certain cases that 

 we can account for their disappearance. It appears to have 

 been sometimes due to a sudden catastrophe, the ejection of 

 mud, or poisonous matter into the medium in which they live. 

 This happens now, and probably has happened before, but in 

 a majority of instances, it is impossible to perceive any exter- 

 nal cause which destroyed them ; and hence, we are left to 

 speculate on probabilities, without being able to arrive at sat- 

 isfactory conclusions. 



MAMMALIA. EQUTJS CABALLUS. 



There is scarcely a question so interesting to the naturalist 

 and historian as that which relates to fossil remains of the 

 horse. The testimony of historians is, that the horse was not 

 living upon this continent at the time of its discovery by 

 Columbus. The testimony of the naturalist is, that the horse 

 lived upon this continent at a period prior to its discovery, 

 its remains having been found first in the miocene, and lastly 

 in the pliocene, in which period it may have become extinct. 

 Its earliest appearance is in the former; and it appears from 

 the discovery of Prof. Holmes, of Charleston, S. C., that its 

 remains are not uncommon in the latter. 



FIG. is. Figure 18 represents the crown of the 



third or fourth molar of the left side of the 

 upper jaw. It has complicated enamel 

 plates, or columns, and is somewhat worn, 

 but by no means an old tooth, as its roots 

 are undeveloped. It is two inches long 

 and an inch thick. It is undistinguishable 

 from the corresponding tooth of the recent 

 domestic horse. It is a deep brown color, and looks like a 

 fossil. 



Figure 19 represents the crown of a tooth of the third or 

 fourth molar, probably the third, of the left upper side. It 



