FOSSIL MARINE UNIVALVES. 425 
The animals of these shells are characterized by their respi- 
ratory organs, which are formed of one or two pectiniform 
gills, with a tube or siphon more or less elongated, for the 
free admission of sea-water to the branchial apparatus. This 
organization is indicated in the shell, either by a notch, or 
by a prolonged tubular canal. All the species are, with 
scarcely any exceptions, inhabitants of the sea, and carni- 
vorous. 
I have selected for illustration of the genus Fusus, a cele- 
brated shell of the Crag, known among collectors as the 
“Hissex reversed Whelk,’ Lign. 133, fig. 4; the spire is 
twisted in the opposite direction to the usual mode, and the 
mouth is consequently to the left of the observer ; the same 
species occurs with the spire in the common direction. The 
shells of the genus Plewrotoma are distinguished by an 
incision, or notch, in the side of the right or outer lip; and 
those of Cerithium, by the form of the mouth, see Lign. 
133, fig. 3. The latter is a very numerous genus, and more 
than two hundred fossil species are enumerated ; it contains 
many elegant forms. The Tertiary strata at Grignon are 
particularly rich in these fossils ; the shells are of a pearly 
whiteness, and as perfect as when recent. Some Cerithia 
are of considerable size; the C. gigantewm is from ten to 
fourteen inches in length. The genus PoTaMIDES compre- 
hends shells closely resembling the Cerithia in form, but 
which are inhabitants of fresh-water.* This is an instance 
of the difficulty which sometimes exists of arriving at 
certain conclusions as to the habits of the mollusks, from 
their testaceous coverings alone. 
The Plastic Clay beds at Castle Hill, Newhaven, and in 
the vicinity of Woolwich, abound in two species of shells, 
* Mr. Woodward informs me that they can only be distinguished 
when fossil, by the absence of varices, or “periodic mouths.” The 
recent species are known to be inhabitants of fresh-water, by their 
dark epidermis, corroded points, and horny multi-spiral opercula. 
