316 



ON THE EXTINCT MAMMALIA OF 



Lines. 



55 



28 

 8i 

 8i 



11 



20 



14 



Space occupied by the molars, 



Space occupied by the three back molars, 



Breadth of third molar, 



Breadth of fifth molar. 



Breadth of sixth molar, 



Depth of jaw below third molar, 



Depth immediately in advance of molars, 



2. A portion of the lower jaw, nearly corresponding with the last, but retaining 

 none of the symphysis. From a young animal. As in the Horse, there appears to 

 have been no canine accompanying the deciduous dentition. The functional series of 

 molars consisted of the temporary molars and the following or fourth one of the per- 

 manent series. The fifth molar has protruded to a degree to have commenced 

 trituration. Within the jaw are seen the anterior three permanent molars, fully 

 developed. 



A socket exists in front of the larger teeth, from which a small premolar has been 

 lost. The anterior pair of temporary molars are retained in the specimen ; the third 

 has been broken away. The former have their crowns nearly worn out, and their 

 triturating surfaces, represented in figure 17, present as much simplicity as in the 

 back permanent molars of the preceding specimen. A tubercle, with its summit worn 

 off, appears at the entrance of the median transverse valley externally. 



The permanent molars have the long crowns characteristic of the true Equidm. 

 The worn triturating surface of the fourth of the series is represented in figure 16. 



The jaw is larger in its proportions than in the preceding specimen, though its 

 greater depth is in a measure due to the accommodation of the long teeth included 

 within the bone. The portion in advance of the molars has been somewhat longer. 



The measurements of the specimen are as follow : 



Lines. 



Space occupied by the premolars and following two permanent molars, . 52 



Depth of jaw below back of first temporary molar, . . . .23 



Distance from first molar to back of symphysis, . . . .14 



Distance from first molar to lateral incisor, . . , . .24 



Width of the constriction of the fore part of the jaw, . . .11 



Breadth of first temporary molar, . . . . . .11 



Breadth of second temporary molar, ..... 10^ 



Length of crown internally of second permanent molar, . . .20 



Length of crown internally of fourth permanent molar, . . .18 



Breadth of fourth permanent molar at triturating surface, . . .10 



3. Fragment of the right side of a lower jaw, containing the last two molars about 



