6 Field Columbian Museum — Geology, Vol. II. 



in number and elongated, or three or four and broad. The ilium may 

 differ in its mode of attachment and the form of both pubis and ischium 

 may differ much. 



Certainly among all these characters, and probably not a few 

 others, there will be no dearth of material for classification. Unfor- 

 tunately there are yet many forms in which we do not know what 

 relations these different characters bear to each other, and until we 

 do, any classification must be provisional. I believe that most 

 herpetological taxonomists will agree with me that the differences 

 between Dolichorhynchops and Brachauchenius are more than generic in 

 value, and I doubt not that differences of equal value will be discov- 

 ered in yet other species when we shall know more about them than 

 we do at present. 



The origin of the Plesiosauria I will discuss in a later paper. 

 For the present, I may say that I believe that their nearest affinities 

 among all reptiles, recent or extinct, are with the Dicynodonts. 



