ABSTRACT

Carina narrow, widening but little from the apex downwards, slightly or considerably curved inwards, with the umbo seated at the uppermost point: S. magnum, however, must be excepted, for in it the umbo is sub-central, and the valve almost angularly bent, as will be described in detail under that species. The apex rarely projects freely; but this is a variable point in the same species; the basal margin is either pointed, rounded, or rarely truncated. The chief character by which this valve can be recognized, as belonging to the genus Scalpellum, is the distinct separation by an angle (see Woodcut, Fig. 1, in the introduction), often surmounted by a prominent ridge, of the tectum or roof, from the parietes, which are either steeply or rectangularly inflected; the lines of growth on these parietes are oblique. A still more conspicuous character is afforded by the part (when present), which I have called the intra-parietes; these give to the valve a pieced appearance, and seem let in, to fill up a vacuity between the upper part of the carina and the terga, and this is their real office; they are separated from the true parietes by a ridge, which evidently marks the normal outline of the valve. These intra-parietes are flat, and they have a striated appearance rather different from the rest of the valve; and the lines o f growth on them are extremely oblique, almost parallel to the inner margins of the valve.