168 GASTEROPODS. 



asitic in its habits a view which, though held by most writers, 

 does not appear to be structurally substantiated. While no 

 doubt the Capulus derived the greater part of its food from 

 excrementitious matter, nourishment from other sources may 

 also have been obtained, and in all probability it was very simi- 

 lar to that of the crinoids and the living Calyptrreidse. Fur- 

 thermore, there does not seem to be the slightest indication 

 that the crinoid was in any manner inconvenienced by the 

 attachment of the gasteropod, except, perhaps, in a few cases 

 where the molluscan shell had encircled the posterio-lateral 

 arms, which were in consequence slightly pressed outward. 

 The only really noticeable effect of the presence of Oapulus on 

 the crinoid is a comparatively shallow depression or groove on 

 some of the ventral plates, marking the position of the shell 

 lip; though in the majority of specimens even this feature is 

 not well pronounced. There are no grounds for the view 

 advanced by Trautschold in regard to Cromyocrinus simplex 

 Trauts. and its adhering -Capulus parasiticus Trauts. from the 

 Lower Carboniferous of Eussia, that the crinoid built a cylin- 

 drical process upon the anal plate as a protection against the 

 sedentary gasteropod. 



Illustrative Examples. In some crinoids, as Gilbertsocri- 

 nus, the plates of the vault are more or less convex or nodose. 

 This nodosity of the ventral plates reaches a high development 

 in such forms as G. tuberosus Lyon and Oasseday. Nearly one- 

 half of the known individuals of this species have a gastero- 

 pod adhering. The specimens illustrate well the adaptation of 

 the apertural margin of the shell to the irregularities of the 

 crinoidal surface ; for it is clearly observable, as first pointed 

 out by Meek and Worthen, that the contact of the gasteropod 

 shell and crinoid is not the result of accidental pressure, but 

 that the mollusk adhered to the surface of the crinoid for a 

 considerable period, as is shown by the sinuosities of the peris- 

 tome corresponding exactly to the inequalities of the surface 

 beneath. In young shells the sinuosities of the apertural mar- 

 gin are comparatively much more pronounced than in older 

 individuals. Many of the latter exhibit much irregularity in 



