190 
ON THE STRUCTURE AND CLASSIFICATION 
differ from him in some important respects, profiting in several cases by the ne^v ma- 
terial which has been added to the collection since the publication of his memoir. 
Every facility for this study was extended to me by my friends, Dr. Woodward, Mr. 
Etheridge, Mr. Davies and Mr. Smith Woodward, to whom I wish to express my 
sincere appreciation. I have also enjoyed much assistance in other collections. Dr. 
Ix-moine, of Rheims, kindly allowed me to examine, Avith the rest of his materials, his 
very interesting series of the Eocene Plagiaulax. Professor Cope generously placed 
his'Puerco collection in my hands for study and comparison. Professor Marsh, hav- 
ing a memoir ui>on the Mesozoic Mammalia in preparation, has kindly permitted me 
to make a careful examination of the type specimens of his various genera, which 
has been of great assistance. The American Triassic genera have also been studied 
from the original specimens in the Williams College and Philadelphia Academy col- 
lections. I did not have an opportunity of examining the types of Amphitherium, 
Amphityhis anA AmpJiilestes, axid wca mvicYi indebted to Mr. Lydekker for the assist- 
ance he has extended in connection with the study of these genera. Influenced by 
these opportunities the purpose of the present memoir has gradually shaped itself as 
follows : 
1 . To present as clearly as the available material alloAvs, the characteristic features 
of each of the Rritish mcsozoic genera, without especial regard to specific characters. 
2. To present the principles upon which the mesozoic mammals, in general, may 
Ik? cla.ssificd into larger and smaller zoological divisions and families, including a com- 
plete generic synopsis. Also, to show their relationships to modern orders of mam- 
mals. 
3. To di.scuss the dentition of these genera in its bearing upon the origin and 
succession of mammalian tooth forms. 
I. THE BRITISH MESOZOIC MAMMALIA. 
I he following description of the British genera is based upon study of the original 
siK?cimens in the case of all except the lower Purbeck genera above mentioned ; also 
StercogmithmaM some of the molars of the Rhaetic Microlestes and Hypdprymnopsis. 
ith the aid of Mr. Lydekker, I have here largely relied upon Professor Owen’s draw- 
ing and descriptions, and tluis have been able to place onlv one of these genera in the 
senes of typical figures given upon plates VIII and IX. The notes upon these genera 
are, therefore, somewhat less complete. In the course of studying the type specimens 
winch, as seen m the accompanying figure (1), are often extremely minute and difficult 
o gmsp, laAc learned to attach great importance to minor characters such, for ex- 
ample, as the basal cusps of the premolars, which prove to have a very important 
bearing uixin the problems of affinities and classification. These cusps present vari- 
ous t egrees o ceAclopment and a variety of shapes, Avhich in every case have fixed 
