212 
ON THE STliUCTlIRE AND CLASSIFICATION 
us far as the tips of the inner roAV of cusps, is worn smooth ; in the second molar, in 
liict, the inner row of cusps is neaidy obliterated. 
This (fissimilar attrition of the crowns of the anterior and posterior pairs of molars 
is a very puzzling fact. It would seem to indicate the presence of two kinds of lower 
molars, the first pair of a trenchant character, the second pair of a pattern somewhat 
similar to that in tr? and m^} 
Notes upon the genera Microlestes, Plagiaulax and Slereognathus are given 
under the next section. 
II. THE CLASSIFICATION AND ZOOLOGICAL RELATIONSHIPS OF THE MESOZOIC 
MAMMALIA. 
Our materials for the purpose of classification are very limited. The greater 
number of genera are represented merely by the mandibular dentition between the 
canine and the coronoid proce.ss, the ends of the jaw being usually fractured or wanting. 
Only four of the Ilritish genera are represented by maxillae, and only two bv both 
the maxillary and mandibular dentition. In the American forms, however, several 
upjM'r and lower jaws have been found by Professor Mar-sh. Limb bones are rare 
and when found arc still more difficult to associate. It follows that the only present 
available ba.sis for classification is the dentition. 
^^'e first observe that the Mesozoic Mammalia divide into two large groups. 
In the first group. A, one of the incisors is greatly developed at the expense of the 
others, and of the canine, which usually disappears ; behind these teeth is a diastema 
of varying width, followed by premolars which are subject to great variations in form 
and numl)er, while the molars bear numerous tubercles. In the second group, 13, the 
incisors arc small and numerous, the canine is always present, and well developed; the 
teeth usually form a continuous series, and the molars bear cusps instead of tubercles. 
These two divisions suggest those which obtain among the modern Marsupials, but are 
in fact much more sharply defined and widely separated from each other. Professor 
Flower^has shown the difficulties which arise from the Diprotodont and Polyprotodont 
divisions of the recent Marsupials, upon the lines drawn by Professor Owen, owing to 
the strong similarity in thestructure of the feet observed between families which upon 
the basis of tooth structure fall into different divisions. Admittiiig the marsupial 
re ationship, it is clear that the genera of the first group are closely related to each 
other and widely .separated from the Diprotodonta by their dental structure which 
Ls very dissimilar and indicates that they probably branched off from the stem of 
the recent marsupials at a remote period, probably the Triassic, (see Appendix). 
Atecte. E„eycl„p.Aia Brlitonlc, ninth edition, 1833, p ^6. 
