SOLARIUM. TRACHELIPODA. 47 
and Neritina of Lamarck. So closely are many of the Littorine allied in form 
to species of the preceding genus, that it is impossible to distinguish them 
without the aid of the operculum, which is horny. The operculum of the 
Trochi is also horny, but has more convolutions than that of Littorine. They 
inhabit the sea-shores of almost all countries. The tropical species are thinner 
than those of northern latitudes. 
Fossil species occur in the Tertiary formations, the Coral Rag, and Inferior 
Oolite. 
Genus XLVI. — TROCHUS. — Linneus. 
Generic Character. — Shell conical; the spire elevated, 
sometimes abbreviated ; aperture more or less transversely 
and obliquely depressed, frequently quadrangular, or trapezt- 
form ; its edge being oblique to the direction of the last 
volution, exhibiting the inferior portion of the columella ; 
base generally flattish, discoidal, or, in some instances, con- 
cave; columella more or less arcuated, and its base trun- 
cated in some species; operculum horny, circular, and 
spiral, with many close-set volutions, and separated by an 
external spiral line; outside frequently covered with a 
horny epidermis. 
Trochus levigatus. Plate VI. fig. 12. Found in the 
Crag pits at Holywell. 
The operculum of this genus is invariably thin and horny, but in the species 
of Turbo it is thick’ and testaceous, while it is distinguished from that of 
Littorine by its more numerous convolutions. The general conical form and 
angulated aperture of the Trochi distinguish them from the Turbines, and 
being destitute of a notch at the base of the columella, they are known from 
the Monodonte. 
The Trochi are numerous, and inhabit the ocean, in almost all quarters of 
the globe ; and many of the tropical species grow to a large size. 
Fossil Trochi are numerous, occurring in the newer formations, namely, 
the Calcaire-grossier, the Greensand, London Clay, the Crag, and some few 
as low as the Lias. 
Genus XLVIT.— SOLARIUM. — Lamarck. 
Generic Character. — Shell subdiscoidal beneath ; spire 
obtusely conical, in some instances of a more lengthened 
conical form ; the lower margin of the body angular, and 
rather sharp ; umbilicus broad and deep, and reaching to 
the apex ; its margin crenulated, and exhibiting the internal 
edges of the superior volutions in the form of a winding 
gallery ; aperture wide, trapeziform, with its angles some- 
what rounded, and the outer lip thin and sharp ; outside 
covered with a horny epidermis, more or less spiral and 
variable in form; outer side flat ; inner side furnished w ith 
an irregular, nearly lateral tubercle, 
