5b4 TRACHELIPODA. ONISCIA. 
Genus XVIII. — PURPURA. — Bruguiere. 
Generic Character. — Shell generally ovate, or oblong ; 
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spire short, mostly grooved, granose, or tuberculate, or ex- 
ternally spinose; aperture generally largely dilated, and 
ovate ; margin of the outer lip usually sharp, and frequently 
toothed within, near the edge; emarginate at the base, 
where it is notched, and ending in a short canal; columella 
generally depressed, often internally acute at the edge, and 
terminating below in a sharp point; operculum horny, pro- 
vided with a lateral nucleus, thinner on the margin next the 
columella. 
Purpura tetragona, Plate IV. fig. 40. Found in the 
Crag near Ipswich. 
The Purpure are oceanic shells, and in a recent state are very numerous, 
occupying a wide geographical range. The Fossil species are, however, very 
limited. 
Genus XIX. — CASSIS. — Bruguiere. 
Generic Character. — Shell ventricose, gibbous, sub- 
trigonal for the most part; body large; spire generally 
very short; aperture longitudinal, narrow ; in some species 
nearly equal to the whole length of the shell, in others pro- 
portionally wider, (in which case the aperture can hardly 
be considered as elongated,) with a more produced spire ; 
base of the aperture terminating in a short canal, which is 
abruptly reflected, and its inner margin acute; columella 
twisted, provided with transverse plaits, or rugose; outer lip 
usually inflected, and spread over the lower portion of the 
body, producing a flattened disk, reaching beyond the edge 
of the lower varix, and internally dentated; and, in many 
species, forming a varix at the completion of each volution. 
Not known whether it is provided with an operculum, 
Cassis bicatinatus. Plate III. fig. 23. Found in the 
Crag at Bawdsey, Sussex. 
The species of this genus are numerous, they have either protuberances on 
the superior part of the volutions, and are decussated, cancellated, grooved 
and striated in various ways. They are oceanic, and inhabit the tropical 
climates, burrowing in the sand at a distance from the shore. 
The fossil specics we few, and these are found in the Tertiary formations. 
Genus XX.— ONISCIA. — Sowerby. 
Generic Character. — Shell oblong-ovate, subcylindrical ; 
broad above, acuminated below, and usually cancellated, 
ribbed or tuberculated; spire generally short, terminating 
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