THE LAMELLIBRANCHS OF THE SILURIAN ROCKS OF GIRVAN. 543 



variabilis of the Lower Carboniferous in shape, but differing from that shell in having 

 the characteristic oblique grooves of Grammysia and an entire pallial line. 



The single specimen in the collection is nearly perfect and has both valves, which 

 resemble each other in all details. 



Genus Conocardium, Bronn, 1835. 

 For synonymy, vide Hind, 1900, Mono. Brit. Garb. Lamell. (Pal. Soc), p. 449. 



Generic Characters. — Shell equivalve, very inequilateral, fusiform or triangular, 

 compressed and truncated posteriorly so as to form a posterior surface. Cordiform 

 and often concave, bisected by a median ridge formed by the closed edges of the valve, 

 from the upper part of which springs the tubular rostrum in those species which possess 

 it. Anterior end triangular and produced, gaping in front. The hinge line straight. 

 Umbones small, pointed, incurved, often carinate posteriorly. An acute ridge passes 

 from the umbo to the postero-inferior angle. Attached to the posterior margin is often 

 a broad shelly process or flange. 



For internal characters vide op. supra cit. p. 451. 



Exterior. — The surface in front of the flange is ornamented with radiating ribs, 

 often obscured by shell growth, and then almost smooth, with smooth concentric lines 

 of growth. Shell thick. 



Observations. — Conocardium is represented in the Ordovician rocks of the West of 

 Scotland by two species, very characteristic of the genus, showing that little or no 

 development took place between Ordovician and Carboniferous times. Both the species 

 have been previously described. 



C. pristis, M'Coy, has the shelly extension or flange to its posterior border so 

 characteristic of C. hibernicum. I have been unable to find evidence of the rostral tube 

 in either specimen, but it is easily left in the matrix, on account of its fragile nature. 



Conocardium pristis, M'Coy, sp., 1846. (PL V., figs. 24-26.) 



Pleurorhynchus pristis, M'Coy, 1846, Sil. Foss. Ireland, p. 71, pi. v. fig. 4. 

 Conocardium pristis, Morris, 1854, Carb. Brit. Foss., 2nd ed., p. 195. 



Specific Characters. -^Shell small, triangular, almost conical when bivalve, 

 moderately gibbose posteriorly. The anterior end pointed, gaping. The lower border 

 descends rapidly and is gently convex. The posterior end adpressed, forming a 

 cordiform surface, the external margin being produced into a shelly process or shield 

 continuous in both valves, and more produced at the inferior angle, measuring as much 

 as the rest of the valve. The hinge-line is nearly straight. The umbones are posterior 

 and terminal, flattened on the posterior surface, incurved, carinated, and contiguous. 



Interior. — Normal . 



Exterior. — In perfect specimens the surface is almost smooth, covered with fine 

 concentric lines of growth. If the surface has been removed, regular radiating ribs are 



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