ORDER I. Atremata (Beecher).—Inarticulate Brachiopoda, with the pedicle passing out between the umbones, the opening being shared by both valves. Pro-deltidium attached to dorsal valves. FAMILIES.—Paterinidae, Obolidae, Trimerellidae, Lingulellidae, Lingulidae, Ligulasmatidae.
ORDER II. Neotremata (Beecher).—More or less circular, cone-shaped, inarticulate Brachiopoda. The pedicle passes out at right angles to the plane of junction of the valves of the shell; the opening is confined to the ventral valve, and may take the form of a slit, or may be closed by the development of a special plate called the listrium, or by a pseudo-deltidium. Pro-deltidium attached to ventral valve. FAMILIES.—Acrotretidae, Siphonotretidae, Trematidae, Discinidae, Craniidae.
Class II. Testicardines (Articulata)
ORDER III. Protremata (Beecher).—Articulate Brachiopoda, with pedicle-opening restricted to ventral valve, and either open at the hinge line or more or less completely closed by a pseudo-deltidium, which may disappear in adult. The pro-deltidium originating on the dorsal surface later becomes anchylosed with the ventral valve. FAMILIES.—Kutorginidae, Eichwaldiidae, Billingsellidae, Strophomenidae, Thecidiidae, Productidae, Richthofenidae, Orthidae, Clitambonitidae, Syntrophiidae, Porambonitidae, Pentameridae.
ORDER IV. Telotremata (Beecher).—Articulate Brachiopoda, with the pedicle-opening, confined in later life to the ventral valve, and placed at the umbo or beneath it. Deltidium present, but no pro-deltidium. Lophophore supported by calcareous loops, &c. FAMILIES.—Protorhynchidae, Rhynchonellidae, Centronellidae, Terebratulidae, Stringocephalidae, Megalanteridae, Terebratellidae, Atrypidae, Spiriferidae, Athyridae.
Affinities.—Little light has been thrown on the affinities of the Brachiopoda by recent research, though speculation has not been wanting. Brachiopods have been at various times placed with the Mollusca, the Chaetopoda, the Chaetognatha, the Phoronidea, the Polyzoa, the Hemichordata, and the Urochordata. None of these alliances has borne close scrutiny. The suggestion to place Brachiopods with the Polyzoa, Phoronis, Rhabdopleura and Cephalodiscus, in the Phylum Podaxonia made in Ency. Brit. (vol. xix, ninth edition, pp. 440-441) has not met with acceptance, and until we have a fuller account of the embryology of some one form, preferably an Inarticulate, it is wiser to regard the group as a very isolated one. It may, however, be pointed out that Brachiopods seem to belong to that class of animal which commences life as a larva with three segments, and that tri-segmented larvae have been found now in several of the larger groups.
Distribution.—Brachiopods first appear in the Lower Cambrian, and reached their highest development in the Silurian, from which upwards of 2000 species are known, and were nearly as numerous in the Devonian period; at present they are represented by some 140 recent species. The following have been found in the British area, as defined by A.M. Norman, Terebratulina caput-serpentis L., Terebratula (Gwynia) capsula Jeff., Magellania (Macandrevia) cranium Müll., M. septigera Lovén, Terebratella spitzbergenensis Dav., Megathyris decollata Chemn., Cistella cistellula S. Wood, Cryptopora gnomon Jeff., Rhynchonella (Hemithyris) psittacea Gmel., Crania anomala Müll., and Discinisca atlantica King. About one-half the 120 existing species are found above the 100-fathoms line. Below 150 fathoms they are rare, but a few such as Terebratulina wyvillei are found down to 2000 fathoms. Lingula is essentially a very shallow water form. As a rule the genera of the northern hemisphere differ from those of the southern. A large number of specimens of a species are usually found together, since their only mode of spreading is during the ciliated larval stage, which although it swims vigorously can only cover a few millimetres an hour; still it may be carried some little distance by currents.
Undue stress is often laid on the fact that Lingula has come down to us apparently unchanged since Cambrian times, whilst Crania, and forms very closely resembling Discina and Rhynchonella, are found from the Ordovician strata onwards. The former statement is, however, true of animals from other classes at least as highly organized as Brachiopods, e.g. the Gasteropod Capulus, whilst most of the invertebrate classes were represented in the Ordovician by forms which do not differ from their existing representatives in any important respect.
A full bibliography of Brachiopoda (recent and fossil) is to be found in Davidson’s Monograph of British Fossil Brachiopods, Pal. Soc. Mon. vi., 1886. The Monograph on Recent Brachiopoda, by the same author, Tr. Linn. Soc. London, Zool. ser. ii. vol. iv., 1886-1888, must on no account be omitted.
(A. E. S.)