188 Messrs. Jones and Kirkby on the 



of Glasgow, is allied to K. costata. It is about the smallest 

 member of the genus ; and it is rare, having as yet oc- 

 curred only in two localities, both of which are north of the 

 Border. 



Localities. — Scotland. Carboniferous Limestone (Lower) : 

 Campbelltown, Argyleshire. Carboniferous Limestone (Up- 

 per) : Linlithgow Bridge, Linlithgowshire. 



10. Kirlcbya rigida, Jones & Kirkby. (PI. III. fig. 18.) 



Beyrichia riyida, J. & K., 1867, Trans. Geol. Soc. Glasgow, vol. ii. 



p. 220 ; 1871, vol. iii. Suppl. p. 26. 



Beyrichia riqida, Armstrong and others, 1876, Catal. W.-Seot. Fos3. 



p. 43. 



Elongate, nearly oblong, compressed, height half the length 

 and less. Dorsal border straight, and not much less than tiie 

 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 

 flat, 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 (tig. 18, b, lateral 

 contour) is elongately oblong, much compressed. Subcentral 

 pit circular, small or obsolete. Surface reticulated, but very 

 often more or less incrusted. Shell moderately thick. Length 



ih~-h 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 end ; 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. 



