288 ON THE EXTINCT MAMMALIA OF 



it probable they belonged to the same individual. They consist of the first, fifth and 

 sixth of the series, and are perhaps not more than a third worn. 



The first of the series, represented in figure 25, plate XVIII, resembles the corres- 

 jionding tooth of the first series ascribed to Protohippus placidus in size, form and 

 proportions. The internal column appears on the triturating surface isolated, as in 

 JBipparion, but it is a question whether at the same stage of attrition it would not 

 have been associated with the antero-median column, as it appears in the tooth of 

 Protohippus placidus. The central lakes are observed to be less gaping than in the 

 latter, and their surrounding enamel is rather more folded. The remnant of the 

 posterior enamel inflection of the crown is seen as a small circular islet occupying 

 the inner back corner of the triturating surface. 



The fifth and sixth molars, of which the latter is represented in figure 26, plate 

 XVIII, exhibit rather more complexity in the folding of the enamel surrounding the 

 central lakes of the triturating surface, than in the first of the series, but otherwise 

 hold an intimate relationship of character with it. The outer part of the fifth tooth 

 is lost, and the remnant of the posterior valley is observed as an oval islet on the 

 masticating surface. 



The measurements of the specimens are as follow : 



Antero-postei'ior diameter. 

 Transverse diameter, 

 Length externally, 

 Length internally. 

 Breadth of internal column, 



2. A second or third molar, worn just sufficiently to exhibit the characteristic 

 arrangement of the enamel on the triturating surface, as represented in figure 27, 

 plate XVIII. The posterior enamel inflection of the crown, as usually existing in 

 Hipparion and Equus, already appears in the masticating surface as an irregularly 

 circular islet. The internal elliptical islet, or section of the internal column, is wide, 

 and exhibits an anterior prolongation towards the anterior median column, as if 

 about to establish an intercommunication as in ProioMp>pus. The measurements of 

 the specimen are as follow : ■ 



Lines. 



Diameter of triturating surface antero-posteriorly and externally, . . 9j 



Diameter of crown at bottom antero-posteriorly and externally, . . 6 J 



Diameter of triturating surface transversely, . . . .8* 



Length of crown along external median ridge, . . . .21 



superior molar. 



5th. 



6tli. 



Lines. 



Lines. 



Lines. 



. lOi 



8 



8 



. Si 





Qi 



. 15 





15 



. 12 



13 



12 



. 2i 



3 



3i 



