1888.] NATURAL SCIENCES OF PHILADELPHIA. 237 



species and the European foi'm the possession by the former of only 

 sixteen eostse, or about half the number ascribed to B. urii by de 

 Koninck. 



Pleurotomaria brazoensis Shumard. 



Pleurotomaria brazoensis Shumard, 1860. Trans. St. Louis Acad. 

 Sci., vol. I, p. 624. 



Pleurotomaria brazoensis f Meek and Worthen, 1866. Geol. 111., 

 vol. II, p. 354. 



The specimens of this species collected do not present much varia- 

 tion. The two peripheral carinse are nearly equal and between them 

 is located the concave band of the sinus. The whorls are orna- 

 mented by sixteen or seventeen strong filiform lines — nine below the 

 lower carina, upon which there are two filiform lines ; three above 

 the upper carina upon which there are two and sometimes three 

 lines; and a single line on the sinal band. Crossing the revolving 

 lines are numerous prominent, equidistant transverse lines which 

 give to the whole ornamentation a peculiarly yet regularly cancella- 

 ted appearance ; between, and parallel to, these transverse raised strise 

 are also from three to six microscopic, yet sharp and distinct raised 

 strice. Meek and Worthen refer with a query to Shumard's species 

 a form from Macoupin county, Illinois, having about twenty-five 

 revolving lines (twelve of which occupy the lower side of the body- 

 Avhorl) instead of thirteen or fourteen as ascribed by Shumard to 

 this species. Shumard says : " surface of volutions ornamented with 

 from thirteen to fourteen rather strong filiform strise which are 

 crossed by sharp transverse strise ; " if by this he intends to convey 

 the idea that this is the entire number of lines including those on 

 the under side of the bo<ly whorl, Meek and AVorthen remark that 

 they " should scarcely entertain a doubt in regard to our [their] 

 shell being a distinct species, since it uniformly has about double 

 that number of revolving strise on the last whorl." In this and 

 some other groups of gasteropods much classificatory importance 

 appears to have been attached to the number of revolving costse ; 

 and sometimes a variation of three or four in the number has been 

 almost the only basis for specific distinction. After a critical exam- 

 ination of a large series of different species presenting these characters, 

 the question has arisen relative to the value of the number of costse 

 as a classificatory criterion. In some gasteropodous groups it has, 

 within certain limits of course, small value ; its exact importance 

 in Pleurotomaria and some allied genera has not as yet been satis- 



