STILIFER. 193 



operculate. The presence or absence of an operculum 

 is, to a limited extent^, a good generic character, although 

 certain allied genera (e. g. Mangilia and Conus) com- 

 'prise species some of which have an operculum and 

 others not. The stiliform spire in the present genus, 

 although remarkable, is not peculiar to it, nor to Odo- 

 stomia or lanthina. Melampus bullceoules has the apical 

 Tvhorls formed in the same mammillated fashion ; and in 

 several genera of the BullidcB the shell exhibits the same 

 feature. These, however, may be regarded as cases of 

 analogy rather than of affinity. The first formed whorls 

 or nucleus of the spire, in many univalves, cease to be 

 occupied by the animal after it has attained a certain 

 growth, being too small for its requirements, — like a 

 householder, who usually moves, once at least during 

 his life, into a tenement larger than the one he at first 

 inhabited. In the case of the Mollusca above referred 

 to, the original and now useless tenement remains affixed 

 to the new one ; but in Bulimus decollatus, some species 

 of Clausilid, and in Truncatella truncatula the topmost 

 story is knocked off and replaced by another roof. Ccb- 

 cumglabrum and C. trachea even undergo a partial meta- 

 morphosis, the shell of each having at first a regular 

 spire, and when this is lost becoming a slightly curved 

 cylinder. The genera Leptoconchus of Riippell and 

 Campidotus of Guettard {Magiliis of De Montfort) also 

 appear to be related to Stilifer in their quasi-parasitic 

 habits. The first-named genus is destitute of an oper- 

 culum, except in its younger state ; the second has an 

 operculum at all ages. The conjecture of the late Pro- 

 fessor D'Orbigny that Stilifej' ought to merge in Etilima, 

 because the latter may likewise be parasitic, has no 

 foundation. It is true that species of Eulima have been 

 found in the stomachs of Holothurice; and the " trepang,'' 



VOL. IV. K 



