1890.] NATURAL SCIENCES OF PHILADELPHIA. 157 



undefinedlonffitiidinal folds, several series of small couspicuous nodes; 

 but these in all examples seem to result from the peculiar nodose or- 

 namentation of Platycrimis 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 nourishment of many 

 of these sedentary gasteropods was derived, in great part at least, from 

 the excrementitious matter from crinoids ; and (3) that the form of 

 the peristome and its marginal configuration, being dependent upon 

 the surface of attachment, have small value as characters for specific 

 distinction. 



The Carbonic species of Capulus in which sedentary habits are 

 positively known from the attachment of the gasteropods to echi- 

 noderms, together with the various species of crinoids intimately as- 

 sociated, are given in the accompanying synoptical table, page 158. 



Range of Variability. Among modern gasteropods attention of 

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

 shell as the result of differences in the local conditions 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 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 com- 

 parisons. Lately Capulus has unexpectedly furnished a very interest- 

 ing series illustrating the range of variation in several species. The 

 comparison is perhaps most striking in the projection of ten speci- 

 mens of Capulus equilateralis as recently ^ graphically represented. 

 The case referred to is only a single one of many to be found among 

 the raollusca. It is very significant in its bearing upon the true 

 basis of species ; and indicates plainly that, in attempting to separate 

 specimens specifically, too much stress should not be j^laced upon in- 

 dividual characters. 



Other Causes of Variation. In connection with variation of species 

 it is of great interest to note the apparent effect of gravitation in 

 altering the form of some gasteropod shells. This phase can be more 

 satisfactorily considered in Capulus equilateralis and C. infundihulum 

 than in most other species, because w'hen attached to the vaults of 



1 Variation exhibited by a Carbonic Gasteropod, Am. Geol., vol. Ill, June, 

 1889. 



