The extinct Spiriferidce are a family characterized by the possession of internal calcareous spires extending from the centre of the shell outwards (fig. 11, 3). These spires, like the shell itself, are frequently silicified, and may be disengaged from the matrix by the action of acid. At other times the shell is imbedded in soft marl, removeable by careful washing, so as to show the calcareous lamina of the spire fringed with hair-like processes, formerly the support of cirri. In the genus Spirifera the shell has a long straight hinge-line, and the flattened area of the larger valve has a deltoid byssal The typical species are characteristic of the paheozoic strata, and have a shell-structure like Rhynchanella. The liassic spe cies (Spiriferina, d'Orb.), have punctate shells, and the byssal opening is closed (at least in the adult) by a thin arched plate or " pseudo-deltidium." In the sub-genus Cyrtia (fig. 11, 4), the hinge-area is ultimately as long as it is wide, and the del tidium is perforated in the centre by a byssal tube ; some of the species have a punctate shell. The genus Athyris (Dal man), not always easily distinguished from Terebratula, has usually a smooth and rounded shell, ornamented with concen tric lamella or wing-like expansions (fig. 11, 5) ; the beak is truncated by a round foramen ; the hinge-area is obsolete ; and the spires are as in Spirifera, with the addition of some further complications near the hinge. There are twenty-five species, mostly from the Devonian and carboniferous rocks. The species of Retzia (King) are still more like plaited Terebra tulce, but have lateral spires ; they range from the Silurian strata to the trial. Unites gryphus (fig. 11, 6), a peculiar De vonian fossil, has a prominent beak, perforated in the young shell by a minute apical foramen ; the hinge-area is filled up by a deeply concave deltidium, on each side of which (but only in some specimens) there is a lateral pouch formed by an inflection of the margin of both valves.
Brackiopoda.
i. Orthis hysterita, L. (cast); Devonian, Rhine.
z. Davidsonia Verneuili, Bouch ; Devonian, Eifel.
3. Strophomena rhomboidalis, Wahl. ; U. Silurian, Dudley.
4. Product* semireticuista, Martin ; Carboniferous, Derbyshire. S. Chonetes striatella, Dalm.; U. Ludlow rock, Herefordshire.
6. C,alceola sandalina, Lam.; Devonian, Eifel.
7. Obolus Apollinis, Eichw. ; L. Silurian, Northern Europe. S. Siphonotreta unguiculata, Echw.; U. Silurian, Britain.
The family Orthidce consists of shells with a straight hinge line, bordered by a flat, narrow area, with a central notch in each valve ; the ventral valve is furnished with articulating hinge-teeth, and the dorsal valve has short processes for the support of the oral arms, which appear to have been horizon tally spiral (as in Atrypa). Between the oral processes there is a central projection for the attachment of the cardinal muscles. Internal moulds of the Orthis (fig. 12, exhibit on the ventral side the single attachment of the adductor muscles in the centre, and on each side of it the cardinal muscles ; these are surrounded by the punctate ovarian spaces and impressions of the large pallial sinuses. The genus Orthis includes 100 species, ranging upwards to the Permian, but it is most abundant in the Silurian rocks. Some of the lower
Silurian species have a round foramen in the " pseudo-delti dium," and are called Orthisince (d'Orb.) Other species in the upper palwozoic rocks have the beak twisted or deformed, probably owing to the attachment of the shell when young Streptarhynchus, King). In Strophomena, Rafin Lepterna, Dalm.), there is a minute byssal foramen when young, of which no trace exists in the adult ; and the deltoid notch is also closed, except the space required to receive the divided cardinal process of the dorsal valve. The oral processes appear to be shifted to the centre of the valve. The shell, when young, is plano-convex, but when it has attained a certain size the valves are bent over to one side or the other, and more or less suddenly. The pallial impressions are the same as in Orthis.
The genus Davidsonia, peculiar to the Devonian limestones, resembles an Orthis attached, like Thecidium, by the ventral valve to corals, and sometimes taking the markings of the body on which it grows, like the oyster and Anomie. The pallial impressions are like those of Orthis, and the form of the spiral arms is indicated by prominences which almost fill up the interior of the shell in aged examples. Some indica tions have been obtained of slender calcareous spires for the support of the arms in this genus ; and also in Koninekia, a small shell from the trias of St. Cassian, in which there are always spiral grooves in the interior of the valves crossed by the impressions of the pallial sinuses.
The anomalous fossil called Calceola sandalina (Lam.) is also peculiar to the Devonian limestones. In shape it resembles Cyrtia, but has no hinge, and neither foramen nor internal processes, except a row of small projections along the hinge line, and two small lateral groups of ridges in the smaller valve. The interior is punctato-striate, but has no recognisable muscular markings.
The Productidce are altogether palwozoic fossils, and most abundant in the carboniferous limestones. Their valves are concavo-convex, the hinge-line straight, and the interior marked with distinct impressions of the muscles for opening and closing the valves, and simple vascular spaces. There are 60 species of Products found in the upper palwozoic rocks, and having a very wide range in North and South America, and from Spitzbergen to Thibet and Tasmania. Some of them are extremely variable in form ; many are armed with long tubular spines, and others completely clothed with short, hair-like processes ; they have no hinge-teeth, and the hinge area is extremely narrow, except in the sub-genus Aulosteges of the Russian zechstein. Products proboscidea has its convex valve prolonged into a tube, as if for the constant supply of respiratory currents. The Permian genus Strophalosia has its valves articulated by hinge-teeth, and covered with long and slender hollow spines ; the shell is attached when young by the umbo of the large valve. Chonetes is distinguished from Producta by a row of spines along the hinge-margin of the convex-valve ; it also has a narrow hinge-area with a covered notch, and small hinge-teeth. There are 25 species in the Silurian and carboniferous, usually of small size, and finely striated.