The species are very numerous; more than 300 extinct forms have been determined.[353] Those figured in _Lign. 125_ are from the White Chalk, and are beautifully preserved; even vestiges of the colour occasionally remain. In a living state, the animal is fixed to foreign bodies by a byssus, or peduncle, which passes through the opening in the beak, or arched extremity, of the shells,[354] The most interesting circumstance relating to these mollusca, is the respiratory apparatus, which consists of two long ciliated tubes, spirally coiled, united at their base, and supported by slender calcareous processes, which are often preserved in the fossils. Thus, in specimens from the soft chalk, the calcareous earth may be removed from the interior of the shell, and the appendages exposed, as in the examples, _Lign. 126, figs. 1, 2_; and in the shells that are empty, these processes occasionally remain distinct, or are coated by a thin pellicle of calcareous spar, or pyrites. [353] See Catalogue of _Terebratulidæ_, published for the British Museum. [354] In the British Museum (Eastern Zoological Gallery, case table A) there are between thirty and forty recent terebratulæ (_T. australis_, Quoy, a plaited species, much resembling _T. fimbria_ of the Inf. Oolite, Cheltenham) attached with their byssi to a block of stone, from Port Jackson, where it was found by Mr. Jukes just below low-water. In the smooth _Terebratulæ_, the laminations of the shell are full of minute perforations, which may be seen by a lens of moderate power; the appearance of this structure, when highly magnified, is shown _fig._ 2a, _Lign. 126_.[355] The Rhynchonellæ (as _Lign. 125, figs. 1, 2_,) do not possess this organization. [355] An interesting Memoir on the Microscopal Examination of Shells has recently been communicated to the Royal Society by Dr. Carpenter. Several species of Terebratula are found both living and fossil, _e.g._ _Terebratula vitrea_, living in the Mediterranean, fossil in Sicily,--_T. caput-serpentis_, recent in the British seas, fossil in the Crag,--and _T. lenticularis_, both recent and fossil in New Zealand. [Illustration: Lign. 126. Terebratula and Spirifer. Fig. 1 and 2.--Upper and under valve of Terebratula carnea. _Chalk; Lewes_: _a_, _a_, remains of the calcareous support of the _brachia_. 2_a_.--Portion of the shell of _Terebratula carnea_, magnified to exhibit the perforations. 3.--Spirifer trigonalis, with part of the upper valve removed, to show one of the spiral processes. (_Min. Conch._) _Mountain Limestone._ ] [Sidenote: SPIRIFER. PENTAMERUS.]