Spirifer (_containing spiral processes_). _Lign. 126._--In the Silurian, Devonian, and Carboniferous limestones there is a profusion of several genera of _Brachiopoda_, whose peculiar forms render them easily recognisable. Among these, the Spirifers are the most interesting, on account of their spiral calcareous processes, which in the recent state supported the ciliated _brachia_, being often preserved. A specimen, in which part of the upper valve of the shell has been removed, and one of the spires exposed, is figured _Lign. 126, fig. 3_. (_Wond._ pp. 735, 736).[356] [356] See a Memoir on the Anatomy of the Brachiopoda, by Professor Owen. Zoological Trans, vol. i. p. 145, _et seq._ All these genera are extinct; they prevail in the oldest fossiliferous rocks, and gradually disappear as we ascend to the newer formations; the last trace of their existence is in the Lias, in which one species has been found. But the _Terebratulæ_ abound in the Lias, Oolite, Chalk, &c., occur in the tertiary formations, and several living species inhabit the seas around Australia and New Zealand. (See _ante_ p. 390.) Rhynchonella, _Fischer_. The "_plaited_" _Terebratulæ_ differ from the typical species (_e.g._ _T. australis_, caput-serpentis, vitrea, &c.) more than even the Spirifers differ, and must be regarded as forming a distinct _family_, Rhynchonellidæ, which will include _Pentamerus_. The shell is not punctate; the arms are spiral, supported only at their origins by shelly processes; the larger valve is _beaked_ acutely, and has a notch within the beak through which the pedicle passes; sometimes the notch is converted into a foramen, by two little plates, (_deltidium_,) as in _Terebratula_. The form of the Rhynchonellæ is tetrahedral. _Lign. 125_. Pentamerus, _Ly._ p. 352.--With the Spirifers, and other Brachiopoda of the Silurian System, some bivalves which, in their general figure, resemble certain species of Terebratulæ, frequently occur. These shells differ in their internal structure from all other genera, in having a septum, or plate, by which their cavity is divided into four chambers; and in one valve the septum itself contains a cell, thus making five chambers, whence the name _Pentamerus_ (_five-celled_). The casts of these shells often have fissures, produced by the decomposition of the septa; and occasionally these cavities are occupied by calcareous spar. Specimens of this kind commonly split into two parts, in one of which two, and in the other three, chambers may be detected; the fifth chamber is the canal of the peduncle. Four species are known, and all belong to the Silurian rocks. Orthis, Leptæna, and Producta form a third family, with horizontal spiral arms, unsupported by shelly processes. _Davidsonia_ is a Leptæna _attached_ by the ventral valve, and the only genus in this family which is fixed by _the shell itself_.