On Trematis, a new genus belonging to the family of Brachiopodous Mollusca

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Abstract
In looking over the valuable collection of Palæozoic fossils brought from the United States by Mr. Lyell, I was struck by a small Brachiopod possessing a combination of structure hitherto unobserved, which entitles it to be placed in a separate genus. The general appearance of the shell is similar to Orbicula , in which genus it has hitherto been placed. It is nearly circular, but slightly broader than long and with an irregular outline. The shell is attached by a ligament which passes through an oblong slit in the lower valve, as in the Orbiculæ; this slit reaches from about the centre of the shell to near the posterior margin. Both the valves are slightly and irregularly convex, giving a depressed form to the shell. The valves are united by a hinge, of which the details cannot be seen in the specimens; but it is probably formed of two diverging lamellar processes in the dorsal valve, for where the shell of that valve has been worn away, we can trace three calcareous plates diverging from the hinge of the dorsal valve, as in the Leptænoid species of Orthis this and in many of the Spirifers. Wherever these plates are found in ,the Brachiopoda, the outer pair appear to be continuations of the teeth or lamellar processes of the hinge; so that the presence of such plates is sufficient to show that the valves played upon a hinge.

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