ABSTRACT
Scutum unknown. Tergum (PI. IV, figs 1 b, c); this is a medium-sized specimen, 0*5 or o-6 of an inch in length; it is rhomboidal, nearly flat, with a straight, slight ridge, running from the apex, at about one-third of the entire width of the valve from the carinal angle, to the sharp basal angle: the ridge is steep on its carinal side, on which side the whole surface of the valve is somewhat depressed. The upper carinal margin meets the occludent margin at somewhat less than a right angle. Internally, a rather small portion of the apex is marked by lines of growth; and close along the upper carinal margin there is a narrow furrow (b), with a ridge parallel and exterior to it; both furrow and ridge are rather variable, and, no doubt, are produced by contact with the edge of the carina. The occludent margin very slightly exceeds in length the scutal margin; the valve is widest nearly across the middle. A slight depression, with just perceptibly angular sides, runs parallel to the occludent margin. The scutal margin is nearly straight; with the exception of this latter single character, scarcely any difference can be perceived between this valve and that of P. glaber.
