fe' 



The Nautilus. 



Vol. XVII. OCTOBER, 1903. No. 6. 



A NEW GENUS OF TROCHIDiE. 



BY AVILLIAM IIKALEY DALL. 



In dredging between Oahu and Molokai in the Hawaiian Islands, 

 in depths varying between 220 and 436 fathoms, sandy and rocky 

 bottom, the U. S. Fisli Commission steamer "All)atross" obtained a 

 number of specimens of a hirge deep-water gastropod shell, occupied 

 by hermit crabs and completely covered by the extended basal mem- 

 brane of a large Actinia. It is not uncommon to find gastropod 

 shells covered by sponges or hydroid zoophytes, commensal with a 

 Paguroid crab, and it often happens that the zoo|)hyte grows beyond 

 the margin of the aperture forming a shield for the growing crab, to 

 their mutual advantage ; since the crab in such cases does not have 

 to seek a new habitation on the ground tiiat tlie old one has become 

 too small for him, while his movements prevent the zoophyte from 

 becoming smothered by the mud, as might happen if the crab sought 

 another domicile and left the old one lying on the bottom. In such 

 cases the lime of the original shell is often gradually absorbed, though 

 the sponge or zoophyte retains more or less of the original form. In 

 the present case, however, the original shell appears to be normally 

 so deficient in lime as to be practically of a flexible, horny consistency 

 and chiefly composed of a rather tough, thick layer of conchioline. 

 In order that it may retain its shape, it is necessary to keep it in 

 alcohol, as in the case of Torellia and some forms of Velutinida. 



