Mr. J. E. Gray on the Genus Ithizochilu9. 477 



the seven requiring notice, the first or Lhwyd's fossil has been 

 shown to be almost wholly undefined ; and it is as little qualified 

 to be the basis of a species as it was before stated to be that of a 

 genus. In the case of L. striatum the difficulties are increased 

 by quoting Parkinson in addition to Lhwyd. No. 2 differs from 

 the assigned characters of Dr. Fleming's fossil in the nature of 

 the direct centre ; and the comparison cannot be extended to the 

 other internal structures. No. 4, Astrea arachno'ides, is a doubtful 

 coral, and does not admit of an identification with any one of 

 those with which it is associated till full information is published ; 

 while No. 8, Cyathophyllum basaltiforme, also differs from L. stri- 

 atum in the composition of its centre, and the comparison must 

 be limited to that portion of the interior. Lastly, Nos. 11, 13 

 and 14 appear to have inseparable corallites ; and it is not known 

 if the published internal compositions exist in the Lit hos trot ion 

 of Lhwyd and Dr. Fleming. 



XXXVIII. — Observations on the Genus Rhizochilus of Steenstrup. 

 By J. E. Gray, Esq., F.R.S., V.P.Z.S. &c. 



[With a Plate.] 



In the Proceedings of the Royal Danish Academy for May 1850, 

 Professor Steenstrup has described a very interesting genus of 

 univalve shells under the name of Rhizochilus, found attached 

 to the axis of Antipathes ericoides. 



Mr. Cuming, knowing the interest I take in the physiology of 

 Mollusca, has most kindly allowed me to examine the three 

 specimens which he has received from Dr. Steenstrup. The shell 

 of this genus while the animal is growing is free, and would be 

 considered in this state as belonging to the genus Rapana, nearly 

 allied to R. papyracea, but of a more solid consistence ; but when 

 the animals have arrived at their full development, two or more 

 congregate together in groups, each animal forming a more or 

 less irregular, opake, white, solid shelly extension of the outer 

 and inner lip, clasping the axis of the coral or the neighbouring 

 shells, or both, and at length entirely closing the mouth of the 

 shell, and firmly attaching the shells to the coral, or to one 

 another, in such a manner that the animal is completely sur- 

 rounded by a solid shelly case having no communication with 

 the outer world but through the case of the anterior siphon of 

 the mantle, which, by the contraction of the mouth of the shell, 

 has been converted into a shelly tube. (See PI. XVII. B. figs. 1, 2.) 



This self-immurement of the animal within its shell has not 

 been described in any other mollusk, and one i3 led to inquire 



