186 Messrs. Jones and Kirk by on the 



three, ridges traverse the valve longitudinally, one either above 

 or across the pit, the others below; the upper and mid ridges 

 terminate in strong spines, which curve upward and forward. 

 A very delicate rim bounds the free margins. Edge view 

 (lateral contour) subcuneiform (tig. 12, b). Shell thin ; sur- 

 face smooth (?). Length -£% inch. 



In some specimens (probably old and worn) the ridges are 

 almost obsolete ; otherwise this species seems subject to little 

 variation. Mr. James Thomson discovered this species. 



Localities. — England. Carboniferous-Limestone series : 

 Steeraway, Salop ; Calees, Cumberland ; Scremerston and 

 Ridsdale, Northumberland ; Holker Park, Lancashire. 



Scotland. Carboniferous Limestone (Lower) : Craigenglen, 

 Scullengour, in Stirlingshire ; Garpel Water, Ayrshire ; 

 Paiston Quarry, East Lothian. Carboniferous Limestone 

 (Upper) : Ravenscraig, Fifeshire. 



8. Kirkbya costata (M'Coy). (PI. III. figs. 13 a, b, 

 14 a, b ; var. fig. 15.) 



Cythere costata, M'Coy, 1844, Syn. Char. Carb. Foss. p. 165, pi. xxiii. 



fig. 11. 

 Kirkbya costata, J. & K., 1866, Ann. & Mag. Nat. Hist. ser. 3, xviii. 



p. 43. 

 Kirkbya costata, J., K., & B., 1884, Monogr. Brit. Foss. Biv. Entom. 



Pal. Soc. p. 89, pi.. 7. fig. 17. 



Subovate or ovate-oblong, flat-sided, rather highest behind, 

 height more than half the length ; thick-shelled, and strongly 

 ribbed with subcoucentric ridges. Dorsal border straight or 

 nearly so, and over two thirds of the maximum length ; 

 ventral border more or less convex ; extremities rounded, the 

 anterior smaller than the other, and both somewhat angular 

 above. Subcentral pit circular, rather above the median line, 

 and showing internally as a raised spot. Edge view (lateral 

 contour) long-ovate with flattened sides (fig. 13 b, 14 b). The 

 extreme anterior portion of each valve is smooth, but from 

 near the centre of that portion spring two strong ribs, one 

 curving abruptly up and the other down, and then passing 

 along the valves rather obliquely and sinuously to near the 

 postero-ventral angle, where they curve to each other and 

 join ; between them are two or three somewhat smaller but 

 similar ribs, also free at their anterior ends, but connected at 

 the other. Other ribs come in below, more or less parallel to 

 the lowest of those just described ; and others come in above 

 and fill up the triangular space between the upper large rib 

 and the dorsal border, the highest being nearly as strong as 

 the two first-mentioned. There is considerable variation in 



