G1STEROPODS. 169 



the lines of growth, which might at first appear to be due to a 

 change of station, but closer inspection shows that this is not 

 the case. When the plates of the ciinoidal vault are nodose, 

 as in Gilbertsocrinus tuber osus the lines of growth in adult 

 shells, contrary to the more usual manner among gasteropods 

 generally, are far from being even approximately parallel to 

 one another ; and in the lip of the shell a sinus, caused by a 

 nodose plate at one period of growth, may be represented in 

 the next by a projecting lobe, which extended into a deep de- 

 pression between the nodes of two contiguous plates. 



In considering the structural pecularities of the Capulus 

 shell, three features the general form, the configuration of the 

 aperture, and the surface makings appear to have been sus- 

 ceptible of considerable modification as the result of the 

 sedentary habits of the mollusk. An examination of a large 

 series of certain species of the genus reveals the fact that the 

 variant tendency in all three of these particulars is much 

 greater than might be supposed; and when the attachment of 

 these gasteropods to foreign bodies is taken into consideration, 

 the causes for such varietal development become manifest. It 

 has been shown that the mollusk doubtless remained fixed 

 throughout a greater portion of life, and that the surface upon 

 which it first settled determined in great part both the form of 

 the shell and the shape of its aperture. When the surface of 

 attachment was flat, as in the vaults of Gilbertsocrinus and 

 Strotocrinus, the molluscan shell was greatly depressed and 

 the peristome ample; but when the foreign body was strongly 

 convex the shell was more conical, with a comparatively much 

 smaller aperture. It has been stated elsewhere that, in regard 

 to the second of the three variant features observable in the 

 calyptraean shell, the margin of the peristome partakes of all 

 the inequalities of the surface to which the gasteropod adheres. 

 Pew of the species attached to crinoids may be said to have 

 true surface ornamentation, for the longitudinal folds or plica- 

 tions in the shell are in many cases due chiefly to the character 

 of the surface of attachment. In some specimens of Igoceras 

 pa bulocrinus (Owen) there have been noticed, in addition to 



