﻿284 Allen's naturalist's library. 



remains of a large member of the present group, which appears 

 entitled to constitute a family by itself. Its leading characters 

 are to be found in the reduction of the molar teeth to two, and 

 of the premolars to a single pair in each jaw. The molars are 

 much longer than wide, those of the upper jaw having three, 

 and those of the lower jaw two, longitudinal ridges ; each ridge 

 having numerous broad blunt tubercles, and the longitudinal 

 grooves so narrow as to be but little wider than the secondary 

 grooves between the tubercles. The single premolar, although 

 simpler, is of the same general structure as the molars. It is 

 noteworthy that the angle of the lower jaw is inflected in the 

 same manner as in the Marsupials. 



Poly mastodon is one of the largest representatives of the 

 Multituberculala, the length of the lower jaw being about four 

 inches. 



FAMILY PLAGIALAUCID^. 



GENUS PLAGIAULAX. 



Plagiaulax ,, Falconer, Quart. Journ. Geo). Soc, vol. xiii., 

 p. 261 (1857); Lydekker, Cat. Foss. Mamm. Brit. Mus., 

 pt. v., p. 196 (1887). 



This last well-defined family of the group under considera- 

 tion is typically represented by the genus Plagiaulax from the 

 upper Jurassic rocks of Dorsetshire, but likewise includes other 

 sfenera from the Cretaceous and lower Tertiary rocks of the - 

 United States, and yet others from the lowest Tertiary strata I 

 of North America and P>ance. 



Agreeing with the preceding family in the elongated form of 

 the molar teeth^ which likewise carry three longitudinal ridges 

 in the upper, and two in the lower jaw, the PlagiaulacidcB are 

 specially characterised by the peculiar conformation of the 

 premolar teeth, which may vary in number from one to four 

 pairs ; the molars being always two. These peculiar premolars, 



