286 trochiDjE. 



Family VII. TRO'CHID^E, D'Orbigny. 



Body spirally twisted into a cone : mantle forming on each 

 side of the head a distinct lobe or lappet : head proboscidiform, 

 furnished with a dentate tongue, the extremity of which is 

 convoluted within the visceral cavity : tentacles long and 

 ciliated : eyes placed on short stalks or tubercles at the outer 

 bases of the tentacles : gills composing a single plume : foot 

 furnished on each side with from 3 to 6 vibracula or ap- 

 pendages resembling tentacles ; operculigerous lobe occu- 

 pying the middle of the upper part of the foot. 



Shell orbicular or conical, and spiral, more or less nacreous : 

 mouth rounded : umbilicus depending in a great measure on 

 the height of the cone, sometimes wanting : operculum horny, 

 thin, circular, and multispiral, with a central nucleus. 



The Trochidce probably live on minute animal and 

 vegetable organisms. From Loven's account of the 

 tongue it seems that the rachis is armed with many 

 teeth, and that each of the pleurse has extremely 

 numerous regularly arranged uncini, which become 

 gradually more slender and simple as they recede 

 from the centre. In Trochus cinerarius there is a 

 large heart-shaped tooth in the middle, and on each 

 side of it five principal or front teeth and about ninety 

 uncini. The sexes are separate. Many of the shells of 

 the typical genus Trochus are extremely ornamental ; 

 and the animals of all are adorned with plumed fila- 

 ments, and with flounces often of resplendent hues. 



Genus I. CYCLOSTKE'MA *, Marryat. PL VII. f. 3. 



Body compressed: head bilobed at its extremity: foot 

 expanded at each of the front corners into a short triangular 



process. 



* Having a circular twist. 



