O. C. Marsh—New Orders of Jurassic Mammals. 289 
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. j 
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 special- 
ized features of modern types, with which they have little real 
affinity. With the exception of avery few aberrant forms, the 
known Mesozoic mammals may be placed in a single order, 
which may appropriately be named Pantotheria. Some of the 
more important characters of this group would be as follows: 
(1.) Cerebral hemispheres smooth. 
(2.) Teeth exceeding, or equalling, the normal number, 44. 
(3.) Premolars and molars imperfectly differentiated. 
(4.) Canine teeth with bifid or grooved fangs. 
(5.) Rami of lower jaw unankylosed at symphysis. 
(6.) Mylohyoid groove distinct on inside of lower jaws. 
(7.) Angle of lower jaw without distinct inflection. 
(8.) Condyle of lower jaw near or below 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 4nd 
Marsupials, at least, were derived. 
Another order of Mesozoic mammals is evidently represented 
by Plagiaulax, the allied genus Ctenacodon, and possibly one 
or two other genera. These are all highly specialized aberrant 
forms, which apparently have left no descendants. This order, 
' which may be termed the Allotheria, can be distinguished from 
the previous group by the following characters : 
(1) Teeth much below the normal number. 
(2) Canine teeth wanting. 
(8) Premolar and molar teeth specialized. 
(4) Angle of lower jaw distinctly inflected. 
(5) Mylohyoid groove wanting. 
Yale College, New Haven, August 7th, 1880. 
