Kirkbya rigida, Jones & Kirkby.

10. Kirkbya rigida, Jones & Kirkby.

(Pl. III. fig. 18.)

Beyrichia rigida, J. & K., 1867 , Trans. Geol. Soc. Glasgow, vol. ii. p. 220; 1871, vol. iii. Suppl. p. 26.

Beyrichia rigida, Armstrong and others, 1876, Catal. W.-Seot. Foss, p. 43.

Elongate7, nearly oblong,L compressed, height half the length and less. Dorsal border straight, and not much less than the maximum length; ventral border more or less concave in most cases; extremities abruptly rounded and nearly alike, the anterior being slightly the smaller. The valves are very Hat, and have two flanges or rims round the extreme and ventral borders; the inner one is usually the larger, projecting beyond the slight convexity of the carapace; the other, more truly marginal, is smaller. They are more or less sparsely denticulated in many specimens. Two narrow ver ­ miform ridges descend from the dorsal border across half or two thirds of the valve, at about a third of the length of the valve from each extremity. Edge view (fig. 18, b, lateral contour) is elongately oblong, much compressed. Subcentral pit circular, small or obsolete. Surface reticulated, but very often more or less incrustcd. Shell moderately thick. Length A- U r inch.

This species varies in relative height and length, but is generally elongated, and much compressed at the sides. The anterior end is usually slightly the smaller, and the posterior is sometimes rather obliquely rounded, projecting most below. The two ribs are curious features, and, at first sight, might almost be looked upon as foreign bodies incrusted on the valves. As a rule, they are placed rather nearest to the anterior cud; but they are not constant in position, and occasionally one of them is much stronger than the other. Seen from below, the carapace, with its ventral flanges, pre ­ sents an elongated figure, tour or five times as long as wide, with parallel or slightly concave sides, and abruptly truncate ends and attenuate corners (fig. 18, b), the ends being fully wider than the centre. In the dorsal view, the sides are decidedly concave and the ends much the widest part of the figure.

We formerly looked upon this species as referable to the genus Beyrichia ; but the general outline of the carapace, its compressed sides, marginal ridges, reticulated surface, and subcentral pit lead us now to place it, without much hesita ­ tion, in Kirkbya , whilst its vertical ridges find an analogy in the median ridges of the next species.

It was discovered by Mr. John Young, of Glasgow, in the Carboniferous-Limestone series (Upper) of Orchard, near Thornliebank, Renfrewshire; and it also occurs in the same position at Kinneil Mill, Linlithgowshire.