THE LAMELLTBHANCHS OF THE SILURIAN ROOKS OF GIRVAN. ■ 529 



markedly prosogyrous. Lunule absent. The hinge plate is edentulous, the ligament 

 small and external. The muscle scars are shallow, and the pallial line entire. Shell 

 thin, either ornamented with fine, regular, concentric lines, or smooth. Ulrich {Filial 

 Rep. Geol. Surv. Minnesota, vol. iii. p. 575) has established the genus Plethocardia 

 for Lower Silurian shells having the external characters of Cardiomorpha. It seems to 

 me very questionable whether this genus is necessary. 



Cardiomorpha prisca. (PL V., figs. 1, la.) 



Specific Characters. — Shell small, very oblique, very inequilateral, gibbose. The 

 anterior end very small and narrow, overhung by the prosogyrous umbones, its margin 

 rounded. The inferior border descends downwards and backwards, and is almost 

 straight. The posterior end broad, its margin bluntly rounded. The hinge line 

 arcuate ; the umbones large, gibbose, raised, and markedly prosogyrous, so that they 

 project some distance in front of the anterior end. No lunule or escutcheon, but a 

 groove behind the umbo parallel to the hinge plate, for the external ligament. 



Interior. — Not examined. 



Exterior. — The surface in the umbonal region is smooth, but near the lower margin 

 are some irregular sulci and lines of growth. 



Dimensions. — Fig. 1, PL V., measures : antero-posteriorly, 18 mm. ; dorso- 

 ventrally, 13 mm. 



Locality.— The Llandeilo series of Ardmillan. 



Observations. — I consider this shell to be very typical of the genus Cardiomorpha. 

 The characteristic groove for the ligament is apparent. The specimen has evidently 

 been somewhat crushed along the dorsal slope, and the ridge in that region is evidently 

 adventitious. 



Genus Edmondia, de Koninck, 1843. 

 For synonymy, see Hind, 1899, Brit. Carb. Lamell. (Pal. Soc), p. 286. 



Generic Characters. — Shell transversely ovate, equivalve, close all round, convexly 

 swollen. Hinge edentulous, simple, erect, possessing a transverse, deeply situated, 

 thickened ridge, separated from the edge of the valve by a smooth groove. No lunule 

 or escutcheon. 



Observations. — The shells of this genus are in shape regular and featureless, and 

 have, as a rule, a simple ornament of more or less rugged concentric lines of growth. 

 In one group there are distinct sulcations, separated by concentric ridges, which may be 

 acute or rounded. 



I have provisionally referred three species of Ordovician shells to this genus ; one, 

 E. silurica, has much the same external ornament as the Carboniferous species 

 E. Maccoyi. 



