TOP-SHELLS. 207 
lappets and tentacular filaments, a pair of true 
tentacles, and eyes fixed on footstalks behind their 
bases; the gill-plume is single; and an operculum 
is always present, spiral in structure, commonly 
circular in shape, and either horny or shelly in 
texture. 
GENUS TROCHUS. 
Even a glance at the British species of this 
genus would show the great diversity that subsists 
in the external form of the shell, from the regular 
pointed cone, in which the whorls do not break 
the uniformity of the outline, as in the beautiful 
T. granulatus and T. striatus, &c., to the tubercled, 
almost hemispheric form of 7. magus, in which the 
swollen and knobbed whorls project like a winding 
staircase round a pictorial ‘“‘ Tower of Babel.” 
The aperture is entire, usually angular, and ap- 
proaching to a four-sided figure, and opens on a 
plane which is oblique with respect to the axis of 
the spire. The interior of the shell is pearly. 
The animal is considerably developed; it is 
furnished with a pair of tapering tentacles, and 
two eyes set at the ends of stout footstalks. Be- 
hind these, on each side, is a large lappet, which 
merges into a broad wing-like expansion of the 
mantle, bearing commonly three, sometimes more, 
tentacular filaments, which are probably delicate 
organs of touch. ‘The foot is oblong, more or less 
lengthened, carrying on its posterior part an oper- 
culum, which is composed of many spires, of horny 
texture. 
Of the sixteen species which are enumerated as 
inhabitants of the British seas, one of the most 
beautiful, and certainly the largest, is the Granu- 
