170 



GASTEBOPODS. 



the undefined longitudinal folds, several series of small con- 

 spicuous nodes ; but these in all examples seem to result from 

 the peculiar nodose ornamentation of Platyerinus hemisphericus 

 with which the univalves are associated. 



It appears, then : (1) that some, if not the majority of the 

 ancient Capuli were stationary during life; (2) that the nour- 

 ishment of many of these sedentary gasteropoda was derived, 

 in great part at least, from the excrementitious matter from 

 crinoids; and (3) that the form of the peristome and its mar- 

 ginal configuration, being dependent upon the surface of attach- 

 ment, have small value as characters for specific distinction. 



Range of Variability. Among modern gasteropoda, atten- 

 tion of late has been called frequently to the variation in the 

 form of the shell as the result of differences in the local con- 

 ditions of station. In the extension of this inquiry to fossil 

 groups many difficulties are met with, among which the most 

 formidable, perhaps, is the inability to obtain enough material 

 for an adequate consideration of the subject. Usually the 



FIGS. 10 and 11. Variation of Capulus equilateralis (Hall) . 



shells of any one species are not abundant locally, nor is the 

 representation from localities, more or less widely separated 

 geographically, sufficient to permit of satisfactory comparisons. 

 Lately Capulus has unexpectedly furnished a very interesting 

 series illustrating the range of variation in several species. The 

 comparison is perhaps most striking in the projection of ten 

 specimens of Capulus equilateralis as recently graphically repre- 

 sented. (Figures LO and 11.) The case referred to is only a 



