Prof. 0. C. Marsh — American Jurassic Mammals. 297 



premolar coincides with this view, and if Hypsiprymnus is a still 

 later descendant, the additional molars, and other herbivorous 

 features, may be the result of this gradual change. 



The few mammals known from the Trias may be placed in two 

 families, Bromotheridce, including the American specimens, and 

 Microhstidce, those of the old world. They are all quite distinct 

 from any of the Jurassic forms, either those found in America or in 

 "Europe. Below the Trias, no Mammals have hitherto been dis- 

 covered ; and none are known with certainty from the Cretaceous. 



Mesozoic Mammals have very generally been referred hitherto 

 to the Marsupialia. An examination of the known remains of 

 American Mesozoic Mammalia, now representing upwards of two 

 hundred distinct individuals, has convinced the writer that they 

 cannot be satisfactorily placed in any of the present orders. This 

 appears to be equally true of the European forms which the writer 

 has had the opportunity of examining. With a few exceptions, the 

 Mesozoic Mammals best preserved are manifestly low generalized 

 forms, without any distinctive Marsupial characters. Many of them 

 show features that point more directly to Insectivores, and present 

 evidence, based on specimens alone, would transfer them to the 

 latter group, if they are to be retained in any modern order. This, 

 however, has not yet been systematically attempted, and the known 

 facts are against it. 



In view of this uncertainty, it seems more in accordance with the 

 present state of science, to recognize the importance of the generalized 

 characters of these early mammals, as at least of ordinal value, 

 rather than attempt to measure them by specialized features of 

 modern types, with which they may have little real affinity. With 

 the exception of a very few aberrant forms, the known Mesozoic 

 mammals may be placed in a single order, which the writer has 

 named PantotJieria} Some of the more important characters of this 

 group are as follows : — 



(1.) Cerebral hemispheres smooth. 



(2.) Teeth exceeding, or equalling, the normal number, 44.'* 



(3.) Canine teeth with bifid or grooved roots.'* 



(4.) Premolars and molars imperfectly differentiated. 



(5.) Rami of lower jaw unankylosed at symphysis. 



(6.) Mylohyoid groove on inside of lower jaw. 



(7.) Angle of lower jaw without inflection. 



(8.) Condyle of lower jaw near horizon of teeth. 



(9.) Condyle vertical or round, not transverse. 



The generalized members of this order were doubtless the forms 

 from which the modern, specialized, Insectivores, at least, were 

 derived. 



Another order of Mesozoic mammals is evidently represented by 

 Allodon, Bolodon, Ctenacodon, Plagiaulax, and a few other genera. 

 These are all highly specialized, aberrant, forms, which apparently 

 have left few, if any, descendants later than the Tertiary. This 



1 American Journal of Science, vol. xx. p. 239, 1880. 

 ^ The genus Faurodon may be an exception. 



