4 FIELD CoLUMBIAN MustuM—GEOLOGY, VoL. II. 
of a more final review of the subject. The present paper contains 
detailed descriptions of Doléchorhyuchops osbornit Williston and Brach- 
auchenius lucasi Williston; a revised description of Cimoliasaurus 
snow? Williston, together with certain descriptions of and remarks. 
upon such other forms from the Kansas Cretaceous as bear more 
or less directly upon the principal species here discussed. As 
will be seen from the list given below, no less than nine distinct 
species of plesiosaurs have been described from the Kansas Cretaceous, 
all of which, except one or two, are autoptically more or less known 
to the writer, together with nearly as many more hitherto undescribed. 
The true generic determination of the most of these species is impos- 
sible at present. So little is known of the real generic characters, not 
only of the American but also of the European plesiosaurs, that, 
unless specimens are very complete, it 1s impossible to correctly 
assign them. Furthermore, there is in many respects such wide 
diversity between the different forms now known that almost every 
species seems rightfully to belong in a different genus. On the other 
hand, in our present ignorance of their value, generic differences can 
rarely be recognized unless one has a considerable portion of the skel- 
eton. Generic determination is, therefore, for the most part, at the 
present time simply guess-work. In the present paper I have, for 
convenience sake, given names to some of these new forms, but the 
generic names are always provisional, and the specific names also 
insome cases. Cvmoliasaurus snowit 1 do not believe is congeneric with 
the type species of the genus; it belongs as well in several other 
genera proposed by American writers. I do not see, however, much 
use in giving new generic names to every form until some razson a’ étre 
can be discovered for them. I have departed from this conviction in 
proposing two new generic names for species herewith described, 
largely because the specimens upon which the names are based are 
more than usually complete, and because there seems to be positive 
characters to sustain the names. 
The full description and illustration of Dolichorhynchops osborni 
will, I trust, aid in the solution of many of these generic problems; 
they will at least furnish a m2ans of comparison for other form; known 
already or to be discovered in the future. 
A second part of this work is to follow soon, I trust. It will 
contain the descriptions and illustrations of two or three other skulls, 
different in structure from those herewith described and from each 
other, together with other important material. 
Iam glad to express my thanks to Prof, Dr. E. Fraas for kind 
