300 ON THE EXTINCT MAMMALIA OF 



their triturating surface communicating with the oblique valley between the median 

 and internal lobes; but in the permanent series the same arrangement is confined to 

 the first molar tooth. It further appears to be the ordinary condition for the central 

 lakes of the first temporary molar to be devoid of ceraentum, while those of the second 

 are partially filled, and those of the third possess the usual amount observed in equine 

 animals generally. In the first and second permanent molars the lakes appear to 

 have been partially occupied with cementum. 

 The measurements of the teeth are as follow : 



1st superior temporary molar. 2J. 

 Lines. Lines. 



Length externally, ...... 4 2i 



Length internally, , . . . . . li 1 



Antero-posterior diameter, ..... 14, estimated. 11 



Transverse diameter, . . . . . 8i 10 



2. A superior temporary molar, most nearly resembling the third of the jaw frag- 

 ment above mentioned, but less worn and of slightly less proportions. The triturating 

 surface is represented in figure 54, plate XVIII, and is not sufficiently worn at its 

 back part to exhibit the usual enamel lines, nor at the inner part to associate the in- 

 ternal and median columns. It evidently presents a likeness to that of the third 

 molar in the jaw fragment before mentioned, as seen by comparing it with the corres- 

 ponding tooth in figure 11, plate XVII. The length of the tooth along the external 

 median ridge is six and a half lines; along the internal column four lines. The 

 antero-posterior diameter of the triturating surface is twelve and a half lines; the 

 transverse diameter nine lines. 



3. A first superior permanent molar, apparently referable to Merycldppus mirabilis. 

 The tooth is much curved, the curvature of the outer side being double the length of 

 the inner curvature. It appears about a third worn, its triturating surface being 

 represented in figure 15, plate XVII. The central lakes have comparatively simple 

 outlines, are capacious, and filled with cementum. The anterior lake, as in the cor- 

 responding tooth of the entire series of Merychipjms insignw, figure 5, communicates 

 with the oblique valley between the median and internal columns of the tooth. 



The measurements of the specimen are as follow : Length following the curvature 

 of the external principal ridge, sixteen lines ; following that of the internal column, 

 seven and a half lines. Antero-posterior diameter of the triturating surface, thirteen 

 and a half lines ; transverse diameter, ten and a half lines. 



4. The fragment of an upper jaw containing the back four molar teeth, represented 

 in figures 8, 9, plate XVII. It belonged to an individual of nearly the same age as 

 that of the complete series of molars referred to Merychippus iusignis, as indicated by 

 the wear of the teeth. 



