In the Galt, or Folkstone-marl, two small species of this genus are to be found in every locality I have visited. They were first figured and described by Mr. Parkinson, under the name of _Inoceramus sulcatus_, and _I. concentricus_ (_Wond._ p. 330, fig. 1 and 3). In most examples the shell is in the state of a white, friable earth, and readily decomposes, leaving patches of iridescent nacre on the casts; but I have seen examples which prove that the originals were of a fibrous structure, like the Inocerami of the Chalk. [Illustration: Lign. 130. Flint, with fragments of Inoceramus. _Chalk. Lewes._ _a._ Marks the section of a fragment of shell, with numerous cavities, occasioned by the depredations of Cliona Conybearei. _b._ Portion of shell partially decomposed, and exposing siliceous, globular bodies, connected by filaments, which are flint casts of the hollows left by the Cliona. ] The shells of the Inocerami, like those of the oyster, and other living mollusca, were exposed to the attacks of some parasite, and perhaps of some Annelid, as the _Nereis_. The shells are often cellular from this cause, and the cavities are found either hollow, or filled with chalk, or, as in the example _Lign. 130_, with flint. In the latter case, upon the decomposition of the shell, the siliceous casts remain in relief on the surface of the flint, as in _Lign. 130, b_. Such specimens are common in the broken flints of the South Downs, and in the shingle on the sea-shore of chalk districts; and their origin would be difficult to understand without this explanation.[365] [365] The Rev. W. Conybeare first ascertained the origin of these fossils, and figured and described them in an elegant Memoir, published in Geol. Trans, vol. ii. first series. Mr. Morris proposes the name of _Clionites_ for the fossil bodies derived from the depredations of the Cliona on the Inocerami and other shells. See Annals Nat. Hist. 1851, and my Pictorial Atlas of Organic Remains. Avicula. _Lyell_, p. 274.--Above fifty species of this genus of shells have been found in the British strata; their general character will be readily understood by reference to the pearl-oyster, (_Avicula margaritifera_,) which is so largely imported for the manufacture of mother-of-pearl ornaments. A remarkable species is found in the Lias, called, from the great disproportion in the size of the shells, _Avicula inæquivalvis_, (_Lyell_, p. 274.) The recent species are inhabitants of warm climates. Our limits will not admit of further notice of the _Monomyaria_, and we proceed to the second division of the plated-gilled mollusca. Dimyaria: _Bivalve Shells, with two muscular imprints_.