FISSURELLA. GASTEROPODA. 65 
Genus IV.— CALYPTRA A, — Zamarch. 
Generic Character. — Shell conical, vertex subcentral ; 
imperforate and acute; base or aperture orbicular, or nearly 
so, its margins sharp and entire; internal cavity provided 
with a lateral salient appendage, or septum, which varies 
much in form in different species : various species have a 
strongly marked, muscular impression, just above the fold 
of the inner lip; in other species, it is situate on the outside 
of the inner cup; but never within it. 
Calyptrea Trochiformis. Plate V. figs. 17 and 24. C. 
rectum. Plate LV. fig. 3. 
The internal appendage varies much in form ; in some it is tongue-shaped, 
emanates from, and is fixed near the summit, with both edges turned in- 
wards; in others it is small, and irregularly triangular, attached by its largest 
side to the internal cavity of the shell; in other species it is cup-shaped; in 
auother division, this consists of a spiral plate, reflected at the upper part, so 
as to form an umbilicus. They are mostly thin shells, with the outside either 
smooth or covered with spinose points, or murications, radiating or concen- 
tric striz or ribs. The outside is invested externally with a thin epidermis. 
They are subject to great variety of form, arising from their being stationary, 
and taking the shape of the substances to which they attach themselves. 
The genus does not consist of many species, they are all Marine, inhabiting 
the shores of America and South Seas; one species, the C. Sinense is a native 
of Britain. 
Fossil species occur in the Calcaire-grossier at Paris, and London Clay. 
Genus V.— PILEOPSIS. — ZLamarch. 
Generic Character. — Shell obliquely conical, posteriorly 
recurved, with an uncinate spiral apex, the volutions serrated 
and rolled inwards ; aperture large, ovate, anterior margin 
shortest, the posterior one large and rounded, inside with 
two elongated, arcuated, muscular impressions, situated 
under the posterior margin; external surface covered with 
a thick, horny, somewhat pilous epidermis. 
Pileopsis vetusta, Plate 1V. fig. 21. Found in the 
Black Limestone, Queens County, Ireland. 
The species of this genus'are few and are marine shells. They inhabit the 
Pacific Ocean, Hast and West Indies, Europe, and one species, the P. 
ungarica, is a native of the British Seas. They are found in deep water, ad- 
hering to oysters and stones. 
Several species are found fossil, in the Tertiary formations, 
Genus V1. — FISSURELLA. — Bruguiere. 
Generic Character, — Shell oblong, shield-shaped, or 
E 
