1(38 PYRAMIDELLID.E. 



is only evolved when feeding : tentacles triangular and folded 

 inwards (not unlike an ass's ears), united at their base by an 

 intermediate membrane ; tips bulbous and ciliated : eyes im- 

 mersed in the skin or outer integument, and placed on the 

 neck a little behind the tentacles. 



Shell forming a cone of various lengths : spire having the 

 first or top whorls sinistral and turned backwards : mouth ex- 

 panding at the base : inner Up very rarely united with the 

 outer lip : pillar usually straight and furnished in the middle 

 with a single tooth or plait : operculum semitestaceous, having 

 a thin flap on the outer side and a short apophysis or process 

 underneath the nucleus of the spire. 



The name and limits of this peculiar genus have been 

 the subject of much controversy. 



The history of its name is as follows. In the Supple- 

 ment to the 4th, 5th, and 6th editions of the ' Encyclo- 

 paedia Britannica' (published at intervals between 1818 

 and 1824) will be found the article a Conchology," by 

 Dr. Fleming. The genus Odostomia is there described 

 as consisting of certain species of marine shells, placed 

 by British writers in the genus Turbo, in which the 

 columella is furnished with a tooth. " The Turbo inter - 

 stincta, unidentata, plicata, Sandivicensis, and insculpta 

 of Montagu are of this genus." This article was sepa- 

 rately republished, with plates, in 1837. Fleming's 

 i Philosophy of Zoology ' (1822) enumerates Odostomia 

 as one of the genera of the " marine Turbonidce ; " and 

 it is therefore most probable that the number of the 

 Encyclopaedia which contained the article " Concho- 

 logy " had then appeared. In 1862 Risso (Hist. Nat. 

 TEur. mer. iv. p. 224) formed the genus Turbonilla, 

 on the MS. authority of Leach, for three fossil species; 

 all are described as longitudinally ribbed, and one of 

 them furnished with a fold. In Turton's ' Enumeration 

 of Marine Shells found on the Devonshire coast ' (1829) 

 Odontostoma was proposed by him as the generic name, 



