In some of the cretaceous strata several extinct species of _Oyster_, _Scallop_, _Arca_, _Tellina_, and other well-known marine bivalves abound; and with them are associated many genera of which no living species have been observed. Among the bivalves that prevail in the English chalk, are three or four kinds of _Terebratulæ_: which are small, elegant, subglobular shells, belonging to a family of which nearly 500 species, referable to several genera, have been obtained from the British strata.[Z] Certain genera are restricted to the most ancient sedimentary rocks, in which they occur in almost incredible numbers; others have a wider range and are met with in the later secondary deposits; while a few are found in the newest beds, and have living representative species in the seas of warm climates. From the immense antiquity of their lineage, these _Terebratulæ_ have been humourously termed the "_fossil aristocracy_." Some of the most common chalk species are figured of the natural size in _Lign. 19_. When living the animal was attached to a rock or other body by means of a _byssus_ or peduncle, exserted through the aperture in the beak or curved extremity of the largest valve.[AA] The shells of the smooth _Terebratulæ_ are full of minute holes or perforations, which may readily be distinguished with a lens of moderate power. [Z] See '_Wonders of Geology_,' 6th Edit. p. 329. [AA] In the Conchological Gallery of the British Museum there is a group of thirty or forty recent _Terebratulæ_ attached to a stone by their peduncles; from Australia. [Sidenote: PETRIFIED OYSTER.] [Illustration: Lign. 20:--Oyster from the Chalk, near Brighton (natural size).] Occasionally the soft body of the mollusk completely silicified--that is, transmuted into flint--is found in its natural position in the shell. A beautiful example of this kind is represented in _Lign. 20_. It is an extinct species of oyster: both valves were entire when I removed the chalk and cleared the specimen; part of one valve has been broken away to expose the petrified body of the animal. I have seen a _Trigonia_[AB] from the oolite of Tisbury in Wiltshire, in which the entire body of the mollusk was transformed into flint, and the _branchiæ_ or lamellated gills were beautifully defined, though converted into semi-transparent chalcedony. [AB] _Trigonia:_ a genus of bivalves, of which there are many extinct species in the chalk and oolite; some bands of Portland stone are an aggregation of _Trigoniæ:_ a few very small species, inhabitants of the seas of Australia and New Zealand, are the only known living forms of this once prevailing type of mollusca. See '_Medals of Creation_,' p. 407. Note II. Page 17. _Wood in Flint._ [Sidenote: WOOD IN FLINT.]