The following is Mr. Owen's description of the peculiar, complex, and extremely delicate teetaccous apparatus, sometimes called ' the carriage-spring' by collectors, attached to the internal surface of the imperforate valve :— " The principal part of this internal skeleton, as it may be termed, consists of a slender, flattened, calcareous loop, the extremities of which are attached to the lateral elevated ridges of the hinge ; the crura of the loop diverge, but again approximate to each other as they advance for a greater or less distance towards the opposite margin of the valve; the loop then suddenly turns towards the perforate valve, and is bent back upon itself for a greater or less extent in different species. When the loop is very short and narrow, as in T. ritrect, Brug., there is but a small tendency towards a reflected portion ; but where the loop is of great length end width, as in T. Chilensie, Brod., T. dorsata, Lam., and T. Sowerbii, King., the reflected portion is con siderable. The loop, besides being :fixed by its origins or crura, is commonly attached to two processes going off at right angles from the sides, or formed by a bifurcation of the extremity, of a central process, which is continued forwards to a greater or Tess extent from the hinge; but it is sometimes entirely free, except at its origins, as, for example, in T. vitrea. This reflected loop, forming two arches on either side the mesial plane, towards which their concavities are directed, I have figured as it exists in T. Chile-neje and T. Sowerbii. It is represented of a similarly perfect form in T. clentata, by 3L De Blainville in his ' 3falacologic ;' and the same apparatus in T. dorsata is very well figured by Chemnitz, by Sowerby, and more recently by G. Fischer de Waldheim. A similar form is also figured in another species of Tercbt at ula by Poll The arches of the loop arc so slender that, notwithstanding their calcareous nature, they possess a slight degree of elasticity and yield a little to pressure ; but for the same reason they readily break if the experiment ho not made with duo caution. The interspace between the two folds of the calcareous loop is filled up by a strong but exten sile membrane, which binds them together, and forms a protecting wall to the viscera : the space between the bifurcated process in 2 Chilensis is also similarly occupied by a strong aponeurosis. In this species the muscular stem of each arm is attached to the outer sides of the loop' and the intervening membrane. They commence at the pointed processes nt the origins of the loop, advance along the lower portion, turn round upon the upper one, and are continued along it till they reach the transverse connecting bar, where they advance again for wards, and terminate by making a half-spiral twist in front of the mouth. It is these free extremities which form the third arm men tioned by Cuvier. These arms are ciliate on their outer side for their entire lcugth, but the cilia arc longer and much finer than the brachial fringes of !Angulo; and except at the extreme ends, which have a slight incurvation, they are uuiformly straight. There is thus an important difference between Lingula and those species of Tercbratula which resemble T. Chilensis in the powers of motion with which the arms aro endowed; since, from their attachment to the calcareous loop, they are fixed, and cannot be unfolded outwards as in Lingula. Owing to this mode of connection, and their ciliated structure, their true nature was much more liable to be mistaken by the early observ ers, though it appears not to hive escaped the discrimination of Lin nscus, who, ns Cuvier has observed, founded his character of the animal of Anemia on the organisation of one of the Terebiotu1e which he included in that genus." The recent species are numerous and widely diffused, and the genus appears to be capable of flourishing in extremely warm and extremely cold regions, as well as in more temperate climates. Thus some of the species have been found in„ the Indian seas and at Java (T. flares cens, Lam., for example), and T. psittarea, brought home from the late expedition by Captain James Ross, R.N., was fished up from a depth of twenty-two fathoms near Felix Harbour, in lat. 70° N., on the east side of Boothia. The average depth at which Terebratela has been found ranges from ten to ninety fathoms. De Blainville has thus subdivided the species :— 'Agars (BrugR?res). Shell subeqnivalve, equilateral, depressed, a little elongated, truncated anteriorly ; the summit menial and poste rior, with no trace of a ligament, but joined at the extremity to a long fibre-gelatinous peduncle, which is supposed to fix it vertically to submarine bodies ; but in the specimen of Lingitta A adcbardii exa mined by Mr. Owen, there was no trace of the adhesion of any foreign body to the end of this pechmele. Muscular impressions Example, lAngula anal ina, The fossil Tercbratakr (properly so called) are extremely numerous, and assist in the identification of strata from the aupracretaceous group to some of the lowest formations in the grailwacke series, both inclusive.
As neither Pent art era, Strygocephalus, Spirifer, Magas, nor Prodacta has living representatives, they are placed here from the structure of their shells, which, judging from analogy, would indicate a brachio podoua construction allied to Terebratala. Indeed De Elitinville retains that name throughout ; but we think the differences of con formation warrant the separation of the fossils above distinguished, as subgenera of the Terebrattaina% They occur principally lu the more ancient fossiliferous beds.
Threidea, Defiance (Thecidism, Sowerby). Do Blainvillo thus describes tho genus :—" Animal entirely unknown, but very probably differing but little from that of Orticala. Shell equilateral, regular,
very inoquivalve, and sufficiently similar to tho Teretrartdre of the latter sections ; one valve hollowed, the heel or hook recurved, entire, without a fissure and adhering ; the other flat opereulifortn, and with out any trace of the internal support Hinge longitudinal ; articulation by two distant condyles, as in the Terebraithr, with a large inertial tooth in the fiat valve fitting between the condyloid teeth of the concave valve.
Example, Tlycidiam radiatum.
The recent species above mentioned is an inhabitant of the Mediter ranean, and found among the common red coral of the Tuscan seas.
Tho fossil species are tolerably numerous, and Sowerby says that those which he had seen appeared to belong to the chalk, and were brought from 31anatricht, and from Orglandea In Normandy.
The recent species have been found at depths ranging from the sur face to seventeen fathoms ; and specimens have been taken in hard coarse sand from four to six inches below the surface of the sand.
Lingula has been found in a fossil state in the Inferior Oolite of Yorkshire, in the Old Red-Sandstone formation, and in other old fos siliferous beds.
Strophomena, Rafinesquo (fossil). Shell regular, equilateral, sub equivalve ; one valve flat, the other slightly excavated : articulation straight, transverse, with a small projection notched or dcntelated transversely. No trace of an internal support.
Example, Strophoracria rugosa.
As Strophomena has no living representatives, at least none yet dis covered, there can be no description of the animal, which is, however, judging from the construction of the shell, most probably brachiopodons.
The fossil genera Plagiostoma, Dtianclora, and Podopais arc placed by De Blainville muter this section. [Pi.smosvogA ; DraNcitons.; l'000rms.] We do not however think that there is such pregnant evidence of a true and entire brachiopodous organisation as to warrant this decided position under the Brachiopods. Indeed Do Blainville himself says that some of the Plagiostomoto are of the family Trrbra tube, and that the others (he instances PIagioa'oma A/anrrllii) are entirely different, and ho allows that these last ought to form a dis tinct genus of the family of Subostraceana. Defiance places Podopaia among the oysters.
* * Shell Unsymmetrical, Irregular, always Adherent.
Orbicula (Lamarck). Shell orbicular, very much compressed; ine quilateral, very inequivalve ; the lower valve very delicate, adhering ; the upper valve patelliform, with the summit more or less inclined towards the posterior side. Fissure of adhesion in the lower valve subeentral. Hinge toothless.
Example, Orbicula lamellosa.
The recent species are found attached to stones, shells, sunken wrecks, &c., and have been found at depths ranging from not far below the surface to seventeen fathoms.
Fossil species are said to have been found in the Lower Greensand of Sussex, in the Speeton Clay of Yorkshire, in both the great and the inferior Oolite, in the carboniferous limestone, and in the Ludlow Rock below the Old Red-Sandstone.
G. B. Sowerby has satisfactorily proved that Lamarck's genus Discina must be expunged, it having been formed from specimens of Orbicula .Norreyica, sent by Sowerby to Lamarck.
Crania (Retzius and others). G. B. Sowerby, who has done so much in the thirteenth volume of tho Linnanui Transactions' to unravel the confusion which had previously been created by authors, gives the following generic characters : Shell inequivalve, generally equilateral, rather irregular, orbicularly subquadrate, and flattish ; the upper valve patelliform, having its umbo or vertex rather behind the centre ; the lower valve attached by its outside, the greater part of it being generally extended over the substance to which it adheres ; and in this respect it differs greatly from Orbicula, which is attached by means of a ligament which passes through a fissure in the centre of the lower valve. There are four muscular impressions in each valve; of those in the upper valve two are in the posterior margin and the other two nearer the centre, but not always very near to each other ; of those in the lower valve two are nearly marginal and rather distant, but the other two are nearly central, and so close together that they appear to form but one : they in general have a small projection between them ; and the whole of the muscular impressions in the lower valve are frequently lost by decomposition in the fossil species, so as to appear only three oblique perforations, as Lamarck has described them.
Example, Crania personate.
The recent species (and this is the only one known) is found adhering to stones and shells at very great depths. It is stated in the Zoologi cal Journal,' by the Rev. M. J. Berkeley, that a specimen of Crania person'ata was taken by Captain Vidal at the depth of 255 fathoms.
There arc several fossil species, mostly from the Chalk.
The species of Brachiopoda in the British seas are not numerous. The following are given in Forbes and Hanky's History of British M olluaca : '— Ten bra t ula (Ilypothyris) psittacca. Undoubtedly indigenous, but very rare.
Tcrebratula Caput Serpcntis. It was first described as British by Dr. Fleming, and has recently been dredged up in considerable num bers in deep water on various arts of the coast.
Tercbratula Cranium. But one specimen of this has been obtained, which is now in the possession of Dr. Fleming.
Terebratula (Mcgathyris) cistellula. This species, which for some time was only known as a Crag-Fossil, has been taken in the Isle of Skye and in the deep-water fishing-grounds of Zetland.
Crania anomala. This species has been taken several times on various parts of the British coast.