4 Field Columbian Museum — Geology, Vol. II. 



of a more final review of the subject. The present paper contains 

 detailed descriptions of Dolichorhynchops osborni Williston and Brack- 

 auchenius lucasi Williston; a revised description of Cimoliasaurus 



snowii 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 is 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 

 in some cases. Cimoliasaurus snowii I 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 raise// cfi/rr 

 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 means of comparison for other forms 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. 



I am glad to express my thanks to Prof. Dr. E. Fraas for kind 



