284 LLOYD'S NATURAL HISTORY. 
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. 
Polymastodon is one of the largest representatives of the 
Multituberculata, the length of the lower jaw being about four 
inches. 
FAMILY PLAGIALAUCID:. 
GENUS PLAGIAULAX, 
Plagiaulax, Falconer, Quart. Journ. Geol. Soc., vol. xiil., 
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 P/agiau/ax from the 
upper Jurassic rocks of Dorsetshire, but likewise includes other 
genera from the Cretaceous and lower Tertiary rocks of the 
United States, and yet others from the lowest Tertiary strata 
of North America and France. 
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 Plagiaulacide 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, 
