156 PROCEEDINGS OF THE ACADEMY OF [1890. 



adhered to the surface of the crinoid for a considerable period, as is 

 shown by the sinuosities of the peristome corresponding exactly to 

 the inequalities of the surface beneath. In young shells the 

 sinuosities of the apertural margin are comparatively much more 

 pronounced than in older individuals. Many of the latter exhibit 

 much irregularity in 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 crinoidal vault are no- 

 dose, as in Gilbertsoerinus tuberosus, the lines of growth in adult 

 shells, contrary to the more usual manner among gasteropods gener- 

 ally, 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 depression between the nodes 

 of two contiguous plates. 



In considering the structural peculiarities of the calyptrtean shell 

 three features — the general form, the configuration of the aperture, 

 and the surface markings — appear to have been susceptible of con- 

 siderable modification as the result of the sedentary habits of the 

 mollusk. An examination of a large series of certain species of 

 Capulus 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 con- 

 sideration 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 Gilbertsoerinus and Stroiocrinus, 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 calyptra^an shell, the margin of the per- 

 istome partakes of all the inequalities of the surface to which the 

 gasteropod adheres. Few of the species attached to crinoids may be 

 said to have true surface ornamentation, for the longitudinal folds 

 or jilications in the shell are in many cases due chiefly to the char- 

 acter of the surface of attachment. In some specimens of Capulus 

 injundibulum (M. & W.) there have been noticed, in addition to the 



