The spirifer received its name from the two highly developed spiral processes in the interior of the shell attached to the dorsal valve. They were hard, like the substance of the shell, and sprang from near the hinge, each diverging outwards to near the border of the valve. They resembled two cork-screws, but the loops were much closer together. These coiled calcareous wires almost filled the hollow of the shell (Fig. 23), and ample support was thus afforded to the filamentous arms. In recent brachiopods, these arms do not always strictly follow the course of the calcareous loops. Among palæozoic genera the case may have been similar, so that the complex calcareous coil of the spirifer may not perhaps indicate a corresponding complexity of the arms. But none of the few recent forms exhibit anything like the coiled processes of the spirifer. The Carboniferous system of Great Britain and Ireland is stated to have yielded between fifty and sixty species of spirifers. Of course, in such a long list the gradations are sometimes very nice, and to an ordinary eye imperceptible, but there exist many marked differences notwithstanding. The general type of the spirifers is tolerably well defined. They had both valves arched outwards, not concavo-convex as in the productus. Their hinge-line, like that of the latter shell, ran in a straight line, and their dorsal valve was raised along its centre from hinge to outer margin, into a prominent ridge, while in the ventral valve there was a furrow exactly to correspond. Most of the species were traversed by sharp ribs radiating from the centre of the hinge-line like those on the surface of the common cockle. But some were quite smooth, retaining only the high lobe in the centre, such as _S. glaber_. In a noble specimen figured by M'Coy[38] under the name of _S. princeps_, the valves are covered with broad plaits that sweep gracefully outward from the centre of the hinge-line. [Footnote 38: _Carb. Limest. Foss. of Ireland_, pl. 21, fig. 1.] [Illustration: Fig. 23.--Spirifer hystericus. _b_, Interior of the same, showing the arrangement of the spiral arms.] The spirifers vary more in form than in external ornament. Some are triangular, others nearly semicircular, others long and attenuated. In some species (as the _S. glaber_), the central ridge is very prominent, taking up about a third of the entire area of the shell, and thus giving it a trilobed appearance. In others (as _S. symmetricus_) it is less marked, and bears a minor furrow down its centre; while in yet a third class (as in some specimens of _S. trigonalis_) the median fold scarcely rises above the ribs that are ranged on each side. These old shells probably anchored themselves to the sea-bottom by means of a thin peduncle, and lived by the vigorous action of those complex fringed arms, whose screw-like skeleton still occasionally remains, and which conveyed to the mouth the animal substances that served as food.