O. C. Marsh— American Surassie Mammals. 345 
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 Pantotheria.* 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.} — 
(8.) 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 order, which the writer has termed the 
Allotheria,t, can be distinguished from the previous group by 
the following characters: 
(1.) Teeth much below the normal number. 
(2.) Canine teeth wanting. 
(3.) Premolar and molar teeth specialized. 
(4.) Mylohyoid groove wanting. 
(5.) Angle of lower jaw distinctly inflected. 
These characters alone do not separate the Plagiaulacide 
and Microlestide from some of the Marsupials, and the facts 
now known seem to prove that they belong in that group, 
where they represent, at least, a well-marked sub-order. 
Of the two families of Triassic Mammals now known, the 
Dromotheride may be placed in the order Pantotheria, and 
the Microlestidw, in the Allotheria. According to present 
evidence, the former were probably placental, and the latter, 
non-placental, and marsupial. 
* This Journal, vol. xx, p. 239, 1880. 
+ The genus Pawrodon may be an exception. 
¢ This Journal, vol. xx, p. 239, 1880. 
