268 ON THE EXTINCT MAMMALIA OF 



horns conjoined ; and the median pair embrace the external pair. The internal 

 columns are smaller, cylindroid, and of secondary importance. 



In the first large upper molar an additional secondary column is developed at the 

 fore part of the crown, and rarely a pair may be found in place of the one. In the 

 last molar the postero-iutemal column generally remains undeveloped. 



The inferior molars, as in the case of the uj^per ones, have long columnar crowns, 

 which are gradually worn away in the course of protrusion, and become inserted into 

 the jaw by fangs only after they are nearly worn out. The crowns are oblong-square, 

 and are composed of an external pair of crescentoid columns and an internal pair of 

 twin-cylindroid columns. The intervals of the columns are filled with cementum. 



The anterior horn of the foremost crescentoid column extends as a wall in a 

 rectangular manner across the front of the crown. The contiguous horns of the 

 crescentoid columns conjoin each other and the anterior twin-cylindroid column. 

 The posterior twin column, smaller than that in advance, and has its back division 

 less well-developed, apparently from its contracted position. In the last molar, where 

 the division of the column just mentioned has more room for development, it is as 

 large as the anterior division. In like manner, from a greater provision of space in 

 the first large lower molar, the anterior extension of the front crescentoid lobe 

 becomes develojaed into a column almost equal to the parent column. 



To the family of Equidm belong the genera Equus, Pi-otohippus, Hipparion and 

 Merychippus. 



EQUUS. 



In the superior molar teeth of the genus Equus the antero-internal column conjoins 

 the antero-median column throughout the length of the crown, except at its unworn 

 summit. In the j^ermanent series, except the first tooth, it is double the size of the 

 postero-internal column, expanding befoi'e as well as behind its connection with the 

 antero-median column ; and in transverse section, as seen on the worn triturating 

 surface, it is broadly elhptical. Examples : Equus caballus, and other recent species 

 of the genus, Equus fossilis of Europe, etc. 



The genus Equus is represented at the present period by half a dozen species, 

 which are all indigenous to the eastern hemisphere. At the time of the discovery of 

 America by Europeans no living member of the genus existed in either continent, 

 though conditions proved to be so favorable that the domesticated species, the Horse 

 and the Ass, subsequently introduced, became widely distributed. The former 

 especially has reproduced to such an extent that it is perhaps as abundant in 

 America as it is elsewhere in the world. It has even also assumed the savage state, 

 in which it is often seen in thrifty herds, having the appearance as if it were indige- 



