490 TROCHIDjE. 



spire of several whorls, aperture entire, usually angulated, 

 sometimes nearly round ; base perforate or imperforate. 



Animal with a large head terminating in a short pro- 

 boscis, and bearing two subulate tentacles, sometimes 

 smooth, sometimes ciliated, and two strong separate eye- 

 peduncles at their outer bases ; between the tentacles are 

 two more or less developed head-lobes, which, however, in 

 some species are nearly obsolete ; next the eye-peduncle, 

 on each side, is a large lappet, continuous with greatly 

 developed side-lobes, bearing usually three, sometimes four 

 or five subulate cirrhi ,• foot more or less lanceolate or 

 oblong, an operculigerous lobe on its upper surface pos- 

 teriorly, bearing a many-spired, horny operculum. Gill 

 very long, linear. Tongue rather long ; each series of 

 denticles composed of a central element flanked by five 

 hooked lateral and numerous narrow accessorial teeth. 



This extensive genus includes a great assemblage of 

 beautiful Mollusks, in which colour, both of soft and hard 

 parts, becomes an important character. Our British species 

 may be divided under three well-marked, subgeneric 

 sections : — 



1. Trochus. Animal with the inter-tentacular lobes 

 very slightly developed. Three lateral cirrhi on each side. 

 Shell more or less pyramidal, imperforate ; to this group 

 belong ziziphinus, conulus, alabastrum, granulatus, striatus, 

 in Meg ran us, exiguus, and Montacuti. 



2. Gibbula. Animal with largely developed inter-ten- 

 tacular lobes. Three lateral cirrhi on each side. Shell 

 tumidly conical, usually perforate — as T. Magus, tumidus, 

 cim rarius, umbUicatus, and, though abnormally, lineatus. 



3. Margarita. Cirrhi five on each side. Shell tumid, 

 thin, usually perforate. T. ffelicinus, imdulatus, and 

 pusillus. 



