6 FieLp CoLUMBIAN MusEUM—GEOLOoGY, 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 andischium 
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. 1 believe that most 
herpetological taxonomists will agree with me that the differences 
between Dolichorhyachops 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 éxtinct, are with the Dicynodonts. 
