O. OC. Marsh—American Jurassic Mammals. 329 
the opposite side of a similar jaw should be shown in another 
specimen, the two may easily be regarded as distinct. This 
may also be the case where upper and lower jaws are found 
separately. Hence, a large amount of material becomes neces- 
sary for even a proximate correlation of the closely related 
forms. 
Plagiaulacide. 
One of the first American specimens discovered resembled 
strongly the minute lower jaws first described by Falconer, un- 
der the name Plagiaulax, and since investigated by Owen, 
Flower, and others, whose discussion of the habits and affinities 
of these peculiar mammals forms a well-known chapter in the 
history of paleontology. Of this genus, only the lower jaws 
were known, and this is one reason for the wide divergence 
of opinion as to the nature of the animals they represent. The | 
lower jaws found in America were regarded by the writer as 
indicating a distinct genus, Ctenacodon, two species of which 
he has since described. 
Among the separate upper jaws found in the Jurassic of 
England were two or three described by Owen, under the 
generic name Bolodon, but with no suspicion that they were 
in any way related to Plagiaulax. From American deposits, 
also, somewhat similar jaws were obtained more recently, and 
as they were apparently quite distinct from Bolodon, they were 
described by-the writer as representing a new genus, A//odon. 
The molar teeth in one specimen resembled those of Plagz- 
aulax, and the writer in his description expressed the opinion 
that Allodon should probably be placed in the Plagiaulacide. 
A natural inference was that Lolodon was the upper jaw of 
Plagiaulax, and Allodon, of Ctenacodon. However this may 
be in regard to the European forms, the specimens now known 
make it clear that the American genera are quite distinct. 
The molar teeth of Ad/odon and Ctenacodon are of the same 
general type, and it is still difficult, if not, impossible, to distin- 
guish them when detached from the jaws. The premolars, 
however, and especially the incisors, differ in the two genera, 
and when well preserved may often be separated with certainty. 
ALLODON. . 
In Allodon, the superior dentition on each side appears to 
be as follows: 
Incisors 3; canine 0; premolars 5; molars 2. 
_ The lower dentition is uncertain, but is probably the follow- 
ing : 
Incisors 1 ; canine 0; premolars 4; molars 2. 
