16 



THE GEOLOGIST. 



Sp. pinguis, Sp. ovalis, Sp. glabra, Sp. Carlukiensis, Sp. Urii, and Sp. lineata ; 

 and it must also be further noted that some of these have hitherto proved 

 exceedmgly rare, and tliat good examples of several others are with difficulty 

 obtained. 



VIII. — Spieifeea DUPLicicosTA. PMUips. PL xii.,* figs. 14, 15. 



Spirifera duplicicosta, Phillips, Geol. York., vol. ii., p. 218., pi. x., fig. 1, 1836 ; 

 and Dav. Garb. Mon., p. 21, pi. iii., figs. 7—10, pi. iv., figs. 3, 5— ll.-j- 



This sheU varies much in external shape, being either transversely sub-rhom- 

 boidal or somewhat longer than wide. The valves are moderately convex, with 

 a more or less elevated mesial fold in the dorsal, and a corresponding sinus in 

 tlie ventral one. The hingeline is shorter than the width of the shell, the area 

 of moderate breadth, and the beak more or less incurved. The valves are orna- 

 mented by numerous radiating ribs, which augment m number at various dis- 

 tances from the beaks by intercalation and bifurcation. The ribs vary, how- 

 ever, somewhat in general appearance in different specimens, being only here 

 and there dupKcose, or having three or four ribs clustered together ; the term 

 duplicicosta is, therefore, very inappropriate, smce many species of the genus 

 present the same peculiarity. Sp. duplicicosta is the largest Scottish spii-ifer 

 with which I am at present acquainted, some specimens measuring twenty-one 

 lines in length by about thirty in width ; and it has also been sometunes con- 

 founded with Sp. striata of Martin — a closely aUied species, but of which I am 

 not acquainted with any well authenticated Scottish example. 



Perfect specimens of this shell are rarely found, although thousands of 

 crushed or broken valves occur in a bed of limestone, four feet in thickness, 

 near Campsie, at Balgrochan, and Corrie Burn, in Stirlingshire. In Ayrshire it 

 is found at West Broadstone, near Beith ; at Craigie, near Kilmamock ; and at 

 Auchenskeigh, near Dairy. In Lanarkshire it has been collected at Brockley, 

 near Lesmahago. It has been also found in West Lothian and Bute. 



IX. — Spirifeea bisulcata. Sowerby. PI. xii., figs. 19 — 25. 



Spirifera bisulcatus^ J. de C. Sow., Min. Con. Tab., 492, figs. 1, 2, 1825 ; 

 Spirifera bisulcata, Dav. Mon. Garb., p. 31, pi. iv., fig. 1, pi. v., fig. 1, pi. vi., 

 figs. 1 — 19, and pi. vii., fig. 4. 



This shell is either transversely semi-circular or obscurely sub-rliomboidal, 

 the valves being almost equally deep or convex. The hinge-Hne is sometimes 

 rather shorter than the greatest width of the shell, with the cardinal angles 

 rounded ; or as long, with angles of variable projection. The area is sub- 

 parallel, of moderate width, and divided by a triangular fissure, the beaks 

 incurved, and at times very approximate. In the dorsal valve the fold is some- 

 what angular, and of greater or lesser elevation, but is at tunes so flattened 

 that it hardly rises above the general convexity of the valve. The suius is of 

 moderate depth. Each valve is ornamented with from thirty to fifty obscurely- 

 rounded ribs, which increase in number by the occasional bifiu'cation or inter- 

 calation of smaller ribs at various distances from the beaks. On the mesial 

 fold they are arranged into three groups, separated by sulci of greater depth, 

 the whole surface being also regularly covered with imbricating strife. 

 The variations in shape presented by this species are quite perplexing, and 

 it is probable that several so termed species will require to be added to its 

 synonyms. Among these we may mention Sp. semicircularis, Phillips, and Sp. 

 crassa, de Koninck, certainly ; and possibly Sp. grandicostata of M'Coy. 



* This plate forms part of the second vohime of the "Geologist." 

 t For list of s.>aionyms, &c., see my monogTaph published by the Palfeontoj^raphical 

 Society, all details not absolutely required hrt\ang been omitted in the present monogTai)h. 



