424 THE MEDALS OF CREATION. Cuap. XI. 
may be distinguished by their discoidal form, the shell 
being coiled up in a nearly vertical plane. There are about 
twenty living species; and sixteen are enumerated as fossil 
in the British tertiary; five occur in the Isle of Wight 
basin, in the localities of the fresh-water genera already 
mentioned; Headon Hill, in particular, yields shells of this 
genus in great abundance and perfection. 
Meuanopsis. Ly. p. 29.—These are spiral univalves, the 
appearance of which will be better understood by the 
figures, than by any description. I allude to this genus 
because a small species is very numerous, with the other 
fresh-water shells, at Headon Hill; and two or more species 
are found in the argillaceous strata of the Wealden (see 
Geol. S. LH. p. 249, and Lign. 132). 
Marine Univatves.—Of the fossil marine Gasteropoda 
there are no less than eighty genera in the strata of the 
British Islands, and the species amount to several hundreds. 
To distinguish the species and genera, reference must, of 
course, be made to works expressly devoted to fossil con- 
chology, as Sowerby’s Mineral Conchology, and Genera of 
Fossil Shells; or to the works of French authors, particu- 
larly those of Lamarck, edited by M. Deshayes, and of M. 
Blainville. The Penny Cyclopedia contains admirable 
notices of fossil shells, under the respective heads of the 
classes, orders, and genera, of the recent Mollusca. 
Buccinum, of which the common Whelk is an example. 
—Fusus, Lign. 133, fig.4. Wond. p. 244. — Pievrotoma, 
Ly. p. 31. Wond. p. 244. — Curiruium, Lign. 133, fig. 
3. Wond. p. 244. — Ancmia, Wond. p. 244. Ly. p. 31. 
—Votuta, Ly. p. 202, fig. 180.— Murex, Ly. p. 164.—Ros- 
TELLARIA, Ly. p. 201.—To the eight genera here enumerated 
a very large number of the marine simple univalve shells 
belong ; and they are principally found in Tertiary strata. 
