PREFACE 



Work on this memoir was begun in October, 1924, and by September, 1926, the actual 

 examination and description of material was complete. The following year was spent 

 in Europe studying Mesozoic mammals as a National Research Fellow in the Biologi- 

 cal Sciences and the chief results have been published in a memoir (Simpson, 1928B) 

 which includes all known European Mesozoic mammals and a few others. In the light 

 of this thorough review of the related European forms, the present memoir was entirely 

 revised and rewritten, bringing it into the form in which it now appears. Although 

 each is a distinct unit, the American and European memoirs are complementary. Repe- 

 tition has been reduced to a minimum, and for a view of the whole subject both should 

 be consulted. 



A number of papers have been published in the American Journal of Science 

 under the general title of "Mesozoic Mammalia" (1925-28). Several of these papers 

 are not merely preliminary as regards the subject of the present work but are also 

 supplementary. It has not been thought necessary to repeat here all the morphological 

 details of, for instance, the American Triconodonta and Symmetrodonta, although for 

 the sake of completeness the various genera and species are redefined and some addi- 

 tions and corrections due to subsequent work are made. 



Of the three known African Mesozoic mammals, TrityLodon finds a place in the 

 European Memoir, Archaeodon in "Mesozoic Mammalia. X," and Brancatherulum 

 in "Mesozoic Mammalia. XI." The one known South American mammal of probable 

 Mesozoic age is considered in the present work. The known Mongolian Mesozoic mam- 

 mals have been treated in one paper by Gregory and Simpson and three by Simpson 

 in the Novitates of the American Museum of Natural History. This publication thus 

 marks the completion of a series of studies which includes all the Mesozoic mammals 

 now known. 



This memoir is based primarily on the magnificent Marsh Collection in the Pea- 

 body Museum of Yale University, the largest collection of Mesozoic mammals in the 

 world, and except for the final revision and rewriting the work has been carried out in 

 the Peabody Museum. 



In its original form, this memoir was presented in 1926 as a Dissertation to the 

 Faculty of the Graduate School of Yale University, in Candidacy for the Degree of 

 Doctor of Philosophy. 



The writer is deeply indebted to Professor R. S. Lull who made this undertaking 

 possible and who has constantly facilitated its conclusion. The trustees of the Othniel 

 Charles Marsh Publication Fund generously provided a fellowship for this purpose 

 for two years and also provided for the illustration of the Peabody Museum material 

 and for various incidental expenses. Dr. J. W. Gidley allowed free access to the very 



