Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/237

Rh assuming great variety of shape (figs. 1, 12 to 20, and 22). All the species lived attached to submarine bodies by the means of a peduncle. Shell structure punctated. Genera Terebratula, Terebratulina, Tertbratella, Wcddheimia, Megerlia, Kraussina, Kinyina, Terebri rostra, Mayas, Mannia, liouchardi t, Plafidia, Aryiope, Cistdla, Reusselaria, Zd- lania, Gwynia, Macandrevia, Diclasma, Meyantheris, Stringoceplialus, Tropidolepis (1). Terebratula appeared at the conclusion of the Silurian period, and continues to be represented up to the present time, but the larger number of genera have had a very limited distribution in time.

Family 6. Thecidædæ.—Shells small, thick, varied in shape, attached by a larger or smaller portion of the shell substance of their ventral valve ; area flat ; deltidium indistinct ; valves articulated ; loop in the dorsal valve folded into two or more lobes lying in hollows of corre sponding shape excavated in the substance of the valve (ligs. 3, 4). This loop, or apophysary ridge, supports the brachial membrane, whose thickened cirrated margin is apparently attached to the inner sides of the grooves ; shell structure punctated. Only one genus, Thecidium. It ap peared in the Trias, and has continued to be represented I up to the present time.

Family 7. Spiriferidæ.—Shells variable in shape, ovate, elongated or transverse trilobed, with the hinge-line at times straight and extended into wing-shaped expansions ; valves articulated, with or without a flattened area in J ventral valve ; animal free or attached during at least a ? portion of its existence by the means of a peduncle, or by muscular fibres issuing from an angular or circular foramen in the beak or area of the ventral valve ; dorsal valve internally furnished with two calcareous spiral processes, connected in different manners, and directed outwards towards the sides of the shell (fig. 5). These processes afforded support to the brachial appendages. This family composes thp following impunctate or punctate genera :. Spirifera, Cyrtia, Spiriferina, Cyrtina, Martinia, Athyris, Merista, Meristina, Retzia, Nudeosjnra, Trematospira, Rhynchospira, Jferistella, Zygospira, Codospira, Rhyndio- trema, Uncites, Ambocoelia, Charionelia, Syringothyris, Uumetria, Suessia, Vetulina (?). The first species belong ing to this family made its appearance during the Silurian period, and the family became entirely extinct in the Inferior Oolite.

{{ti|1em|Family 8. Rhynchonellidæ.—Valves articulated, very variable in shape, more or less trigonal, often trilobed or ovate, smooth or plicated (fig. 2) ; foramen beneath a usually produced and pointed beak, completed by a d jltidium at times concealed ; brachial appendages fleshy and spirally rolled, flexible, and supported only at their origin by a pair of short-curved shelly processes, or throughout by two broad spirally- coiled lamella} (these spires are vertical, closely appressed, and directed towards the centre of the valve) ; shell structure fibrous and impunctate. This family composes the following genera : RhyncJiondla, Atrypa, Eatonia, Lcptoccelia, Brachymerus, Anastrophia, Ldorhynchus, Camarophoria, Rhynckopora, Rhynchondlina, and one or two others. The first species appeared during the Silurian period, and representatives of the family have continued to the present time.}}

Family 9. Pentameridæ.—Shells ovate, somewhat pentagonal ; valves articulated, without hinge-area ; foramen angular; no deltidium; inside of ventral valve two contiguous vertical septa of greater or lesser length, which coalesce into one median plate, and then diverge to form the dental plates, enclosing a triangular trough-like chamber. In the interior of dorsal valve are two longitu dinal septa of variable dimensions, to which the socket walls converge and which they join, forming two more or less de veloped and inclined plates, to the produced extremities of which were no doubt affixed the fleshy spiral labial append ages. Shell structure impunctate. Genera Pentanurus, Pentamerella, and perhaps one or two others. The species of this family are limited to the Silurian, Devonian, and Carboniferous periods.

Family 10. Strophomenidæ (figs. 6, 7).—Shells semicircular, transverse, or elongated; valves usually concavo-convex, regularly arched, geniculated or depressed, so that the valve which is convex in some species is concave in others, and vice versa ; hinge-line long, straight ; area in ventral valve flat, with a fissure partly arched over by a pseudo- deltidium, whfle the extremity of the beak is either entire or perforated by a small circular foramen. In the dorsal valve the projecting bifid cardinal process fills up almost the entire cavity of the fissure that may not have been arched over by the pseudo-deltidium of the opposite valve. Valves sometimes uniformly convex, the dorsal one sometimes depressed with an area divided by a triangular foramen. In the interior of the dorsal valve a small, simple, projecting cardinal process is situated between prominent socket walls, to the inner extremities of which were (probably) attached the brachial appendages. Genera Strophomena, Streptorhynchus, Strophodonta, JLeptcena, Ortkis, Orthcsina, Skenidium, firachyprion, Uisccelosia, Meekella, Davidsonia (]), and several others. The first species appeared during the Silurian period, and the last in the upper Lias. It may, however, be necessary to group the genera provisionally placed in the Strophomenidoe into one or two families or sub-families. A family Orthidce might be established. Strophomena differs from Ortkis in having a closed fissure, and the cardinal process bifid or trilobed, while in Orthis it is generally formed of one piece. In Strophomena it is situated directly between the dental sockets, or has between them and it a small prominent ridge, or brachial process ; for this last is scarcely developed, where it exists, and forms a marked contrast to what we find in the same valve of Orthis. There are also four more or less distinctly defined adductor depressions, which are longitudinally parallel to each other, and separated by ridges, while in Orthis these four divisions are placed in pairs one above the other.

Family 11. Productidæ.—Shells more or less concavo-convex, oval, semi-oval, or angular and generally auriculated; the hinge-line straight, with or without teeth and sockets for the articulation of the valves (fig. 8) ; surface of ventral valve or hinge-line more or less furnished with tubular spines, sometimes of considerable length ; no calcareous processes for the support of the brachial appendages ; shell structure perforated by canals ; cardinal process prominent, bilobed or trilobed. Under this a narrow longitudinal ridge generally extends to about half (or more) of the length of. the valve, and on each side are seen the ramified dendritic impressions, which may be attributable to the adductor muscle. Outside, and in front of these, are the two reniform impressions so characteristic of the family. Genera Produdus, Strophalonia, Aidostegts, Chonctes, Productdla.-^ The Produdidas made their first appearance during the Silurian time, and became extinct at the close of the Palaeozoic period. Assuming that the reader is acquainted with the geo logical divisions into which the earth s crust has been grouped, it may be observed that the Brachiopoda, after the Trilobites, occupy the most important place in the Cambrian or Primordial fauna. Thus, in 1871, out of 241 species known to Barrande as composing the animal kingdom of that period, 179 are referable to the Trilobites and other Crustaceans, 28 to the Brachiopoda, while 34 species would be di% T ided between the Annelids, Pteropodes, Gasteropoda, Bryozoa. Cystidians, and Spongida. Sub sequently to these researches several additional species of 