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Show 357 mented by small linear radiating costte, which are crossed by flue concentric striae and stronger marks of growth. Near the anterior end of the shell, the radiating costae are a little larger and much more widely separated than farther back. Length, 0.75 inch; height, 0.47 inch; breadth or convexity, 0.40 inch. I am not well enough acquainted with the hinge of this little shell to be quite sure that it is congeneric with the form on which I proposed to found the genus Orammatodon in the Palaeontology of the Upper Missouri. One of the casts shows a little of the impression of the hinge in front of the beaks, with apparently four or five small teeth, or denticles, ranging obliquely forward and upward. The posterior muscular impression, as seen in this cast, shows no traces whatever of the lamina connected with that of the posterior adductor, such as exists in Cucullwa. Locality and position.- Komooks, Vancouver's Island; cretaceous. Genus? ARCA, Linn. ARCA? EQUILATEBALIS, Meek. Plate 2, figs. 6 and 6 a. Arca ( CucuUcca f) equilateralis, Meek ( 1857), Trans. Albany Institute, iv, 40. Internal cast of medium size, equilateral, transversely oblong- oval in form, gibbous in the central and umbonal regions; anal margin obliquely truncated; anterior side subtruncated; base nearly straight along the middle, and rounding up abruptly at the extremities; dorsal outline sloping from the beaks; hinge less than the length of the valves, and parallel to the base; cardinal area comparatively spaall; beaks central, moderately elevated, and rather gibbous, but somewhat flattened on the outside, and incurved at right angles to the hinge; muscular impressions shallow. Surface ( of cast) retaining faint traces of small radiating costre, or stria?. Length, 1. G8 inches; height, 1.02 inches; breadth or convexity, 0.80 inch. This species will be readily distinguished from any other with which I am acquainted, resembling it in other respects, by the central position of itsJbeaks. None of the specimens show the hinge, or surface- markings, though faint traces of radiating lines, or striae, seen on the internal casts, show that it was probably marked by small radiating costae. It is not easy to decide from the specimens in the collection whether it is an Arca or a Cucullcea, or, indeed, with certainty, whether it belongs even to the Arcidce. There is on the cast a shallow, obscure depression near each posterior muscular scar; but these depressions seem not so distinct as those always seen on casts of Cucull< eay while none of the specimens show the hinge, though there appears to be a small cardinal area. Locality and position.- Nanaimo?, Vancouver's Island; Cretaceous. |