GASTEROPODS. 167 



pod ; and to suggest that the mollusk was, in all probability, 

 stationed on the echinoderm for a protracted period, perhaps 

 even for the greater portion of its life. But notwithstand- 

 ing the fact that the univalve was almost invariably situated 

 over the ventral aperture, and that this opening was recog- 

 nized as the anus, these writers do not seem to entertain for a 

 moment the idea that the gasteropod may have been nourished 

 upon the refuse matter from the crinoid. The latter view is 

 now favorably received by most paleontologists. In every 

 instance of the several hundred specimens lately examined, 

 the ealyptnean covers the anal opening of the crinoid ; and, 

 so far as observable, it is always the anterior portion of the 

 molluscan shell that is directed to the vault aperture. In those 

 examples where the shell has been removed, its impression 

 made on the ventral surface shows that the anterior margin of 

 the peristome was at the edge of the opening in the dome, a 

 position that would have brought the mouth of the mollusk 

 directly over the anus of the crinoid. From an examination 

 of the concentric markings made by molluscan shells on the 

 tests of Strotocrinus and Platycrinus, it appears that the for- 

 ward end of the Capulns was always stationary at the mar- 

 gin of the dome opening ; and that, as the growth of the shell 

 continued, the posterior portion was removed farther and far- 

 ther from the ventral aperture of the crinoid. 



The food of recent crinoids consists chiefly of animal- 

 cules and microscopic plants, and the living Calyptneidse sub- 

 sists on food of a similar nature. From analogy it might be 

 inferred that the food of fossil crinoids and mollusks must have 

 been similar to that of their modern representatives. So far as 

 the echinoderins are concerned, there seem to be no serious 

 objections to this inference. But with the univalves their 

 position through life indicates that their sustenance was, in 

 great part at least, of a somewhat different character. 



The anatomy of the crinoid and the position of the mol- 

 luscan shell are not in accord with the supposition that the 

 calyptraean may in any way have been nourished on the food 

 of the crinoid. This would imply that the gasteropod was par- 



