286 PROCEEDINGS OF THE ACADEMY OF [1887, 



Bolodon has conical tubercular instead of trenchant 2:)reniolars. In 

 Plagiaulax the tubercles are irregular crenatious of the border of the 

 basin shaped crown. In Bolodon they are minute, shai'ply defined 

 cones arranged in straight rows with a deeply worn groove between 

 them ; this pattern is like that of Tritylodon with two rows of tubercles 

 instead of three. 



The general likeness between Bolodon and Tritylodon is very 

 striking in spite of the great discrepancy in size. The latter genus 

 is perhaps synonymous with Ti^ifjfbjphus* (Fraas) and forms the type 

 of the family (3) Tritylodontidae^ (Cope), characterized by the ab- 

 sence of trenchant premolars and the presence of upper molars with 

 three parallel rows of tubercles separated by grooves. The Poly 

 mastodo7itidce (Cope) forms a fourth, more recent family\ 



The position of Stereognathus, Chirox and Meniscoessus is un- 

 certain, although they probably belong to this Sub-Order. Menis- 

 coessus stands nearest the Plagiaulacidae, and Chirox intermediate 

 between the Bolodontidae and Tritylodontidae. 



II. Second Group. 



1. JURASSIC MAMMALS. 

 Among the second group of mammals I find that the molar 

 pattern forms an advantageous starting point for classification. 

 Contrary to the usual statement, the premolars are invariably unlike 

 the molars'^, but the whole dental series, as well as the mandible, 

 are in a marked degree correlative in structure and in most instances 

 distinctly specialized for certain kinds of diet. Some of the molar 

 patterns with the correlated structures, prevail in a number of genera 

 which we may group in a family, without knowing exactly where to 

 place it in the zoological scale. The molars of other genera are 

 transitional in structure between two distinct types. Other genera 

 again are entirely isolated in their molar structure where no allied 

 forms have been discovered. This classification by families is, of 

 course, a temporary one, subject to change as the genera become 

 more fully known. The /Stylo do ntidae form the only fidly defined 

 family. 



4. See Neuniayr. Neucs Jahrhucli fiir Min. u. Pal. 1884, p. 279. 



1. Am. Naturalist, 18S4. 



2. Phascolotherium forms an apparent Init not real exception, since the pre- 

 molars have probably disappeared. In Diplocynodon, (Marsh, loc. cit. Plate x), 

 the premolars when viewed upon the inner surface are very distinct from the 

 molars. 



