[375] _Unio Valdensis_ resembles in form the Mexican species, U. Panacöensis (River Panaco), but is probably more nearly allied to an unnamed Australian species of which Mr. G, Sowerby has numerous examples. I shall reserve my remarks upon the important aid these fossils afforded in the determination of the fluviatile origin of the Wealden, for our _Excursion to Tilgate Forest_. [Illustration: Lign. 132. Cyclas and Melanopsis. _Wealden; Sussex._] Cyclas. _Wond._ p. 404. _Ly._ p. 28.--Another genus of fresh-water bivalves is termed _Cyclas_, of which there are ten species in the Wealden formation: and, with the exception of four or five recent forms, which occur in the tertiary fresh-water strata, none others have been found in England.[376] The shells of the genus Cyclas are oval, transverse, equivalved bivalves, with the hinge-teeth very small: the substance of the shell is thin and fragile; the figures in _Wond._ and _Ly._ accurately represent the appearance of the fossil _Cyclades_ of the Wealden, and tertiary strata. Entire layers of two or three species of these shells occur in the argillaceous deposits of the Wealden, generally in a friable state, but from among the masses of crushed shells, perfect specimens may be obtained, and sometimes with the remains of the epidermis and ligament. The hard stone, termed calciferous grit, in the neighbourhood of Hastings, Tilgate Forest, Horsham, and other places in the Weald of Sussex, abounds in casts of the same species, associated with the _Uniones_, previously described. In the cliffs on the southern shores of the Isle of Wight where the Wealden beds emerge, and also in the Isle of Purbeck, these shells are equally abundant. Together with the _Uniones_, they occasionally appear in the limestone, called Sussex Marble; and in the Isle of Purbeck there are beds of limestone wholly composed of bivalves belonging to these two genera, and presenting, in polished slabs, markings formed by sections of the enclosed shells. [376] _Cyrena_, is a genus so nearly related to Cyclas, that it is difficult to distinguish them, and it will be convenient to retain only the former name. FOSSIL PTEROPODA. [Sidenote: PTEROPODA. GASTEROPODA.] In the Ludlow strata there are found small fragile elongated conical shells without chambers, which are supposed by Professor E. Forbes to be identical with a recent genus of pteropodous mollusca, common in the Mediterranean, called Creseis. They seldom exceed two inches in length. Of another genus, named Conularia, six Species have been discovered in the Silurian formation.[377] [377] See Geol. Trans, second series, vol. vi, p. 325. [Illustration: Lign. 133. Fossil Shells of Gasteropoda. Fig. 1.--Paludina fluviorum. _Wealden._ 2.--Limnæa longiscata. _Tertiary. Isle of Wight._ 3.--Cerithium lapidorum. _Tertiary, Grignon._ 4.--Fusus contrarius. Crag. Essex. ] FOSSIL SHELLS OF GASTEROPODA.