5. Kirkbya plicata, Jones & Kirkby.

(Pl. III. figs. 9 and 10, a, b.)

Kirkbya plicata, J. & K., 1867, Trans. Geol. Soc. Glasgow, vol. ii. p. 221; 1871, vol. iii. Suppl. p. 28.

Kirkbya plicata, Armstrong and others, 1876, Catal. W.-Scot. Foss, p. 45.

Kirkbya plicata, Kirkby, 1880, Q. J. G. S. vol. xxxvi. p. 588.

Ovate to subovate, compressed, and considerably over half as high as long. Dorsal border straight, and two thirds (or more) of the maximum length; ventral border boldly arched; anterior extremity rounded and decidedly the smaller, sloping below; posterior extremity boldly and obliquely rounded. The subcentral pit is deep, transverse, and usually placed rather posteriorly. A narrow but prominent rim bounds the valves, and two sinuous ribs of equal size to the rim cross the valve from their point of junction near the antero-centre to near the hind margin or the postero-ventral angle, wdiere they again approach very nearly together. These ribs divide the valve into dorsal, central, and ventral areas. The edge-view (lateral contour) varies in different specimens from flatly lenticular to subcuneiform (fig. 10, b). Surface smooth in most of our specimens, but in a few cases finely reticulated. Length 1/23 inch.

Some few examples of this species show a third rib not far from the ventral border (fig. 9). The subcentral pit varies in size; sometimes it is not seen at all, and occasionally it takes more the form of a sulcus.

K. plicata was discovered by the late Mr. Charles Moore, of Bath, in the Carboniferous Limestone of Backwell, Somerset, where it appears to be not at all rare.

Localities.—England. Carboniferous Limestone: Backwell, Charterhouse, Weston-super-Mare, in Somerset.

Scotland. Calciferous Sandstone: Randerstone, Fife; Craiglockhart Quarry and Camps, in Edinburghshire; Larriston Quarry and Penton Bridge, in Roxburghshire. Carbon ­ iferous-Limestone series: Campbelltown, Argylesbire; White ­ field New Quarry, Peeblesshire.