^2, Studies in Geology, No. 3 



spaced threads in vohnsta, and of closely set cords or ribs in 

 abnipta. The point of smallest whorl-diameter seems to be 

 lower on the later whorls of abnipta, with the surface curv- 

 ing outward to the suture instead of vertically, as in robnsta, 

 but a larger amount of material might show inconstancy in 

 that feature. Both these forms, however, dififer in this 

 point from cliarana, in which the suture is at the apex of a 

 distinct notch, the posterior surface of the whorl inclining 

 toward it at an appreciable angle. Charana differs further 

 from both in the keel, which is somewhat more solid and less 

 attenuate, although none the less sharp. The sculpture of 

 charana is similar to that of abnipta, but the threads seem 

 to be more distantly spaced than in that variety. 



All of the species named are in the Hopkins collection 

 save TiirntcUa iiica Grzybowski, whose relationships are 

 discussed here on the basis of Gryzbowski's figure and 

 description. It is somewhat unsatisfactory to work with 

 figures as poor as those published by Grzybowski, but in 

 this case the characteristics of inca seem to be sufficiently 

 well defined to allow conclusions from the data available. 



It is only natural to find that those nearly related species 

 which are represented by an abundance of material show 

 puzzling" intergradations, and a separation of such into 

 groups best conforming to the limitations of a species has 

 not been always as clear a matter as the exactness of descrip- 

 tory or diagrammatic dift"erentiation might suggest; how- 

 ever, the writer has endeavored to select as dividing points 

 those characteristics which promise to serve best to make 

 future study of the fauna clear, doing so in full apprecia- 

 tion of specific values as well as arbitrary matters of distinc- 

 tion. Thus although intergrading forms may present diffi- 

 culty, it is believed that the characters emphasized in this 

 presentation will give an intelligible index to the placing of 

 those forms which belong in the general group. 



