Kirkbya plicata, Jones & Kirkby.

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.