SOLENOMYA COSTELLATA. 443 



with a bluntly rounded margin. The posterior border is arched, rapidly depressed 

 posterior to the umbones, and very nearly as long as the sliell. The umbones are 

 small, compressed, elongate, not elevated, excavated behind by the escutcheon, 

 and placed in the posterior third of the valve. The valves are much more convex 

 from above downwards than from before backwards, and there is a well-marked 

 broad, shallow compression about the middle of the valve, which shows as a slight 

 sinuosity in the ventral border. The greatest gibbosity of the valves is sub- 

 umbonal and just below the dorsal border, the shells becoming gradually com- 

 pressed between this line and the ventral margin. 



Inferior. — The anterior adductor muscle-scar is large and pear-shaped, remote 

 from the margin ; the posterior scar is deep, ovate, bounded in front by a well- 

 marked ridge. Pallial line entire. In casts, above the anterior adductor scar, 

 between it and the hinge edge, is a groove which corresponds with a thickening of 

 the edge of the valve. The internal surface of the shell was almost smooth, with 

 very irregular, almost obsolete, concentric grooves and very fine obsolete radiating 

 lines, most marked in the anterior part of the valve. 



Exterior. — The surface is glossy ; concentric markings are almost entirely 

 obsolete, but the shell is crossed by many flat radiating ribs, fairly close together 

 in front, and becoming more widely separated as they pass towards the middle of 

 the valve, but closer again towards the posterior end. These ribs have rather 

 prominent edges, often with a shallow linear groove between the narrower ones 

 towards the posterior end ; but in the broader anterior ribs the linear groove is 

 replaced by a somewhat hollowed surface. Shell thin over the anterior two thirds, 

 rather thick at the posterior end. 



Dimensions. — The specimen in the Woodwardian Museum, figured as 

 8. primseva by M'Coy, measures — 



Antero-posteriorly . . . .45 mm. 



Dorso-ventrally . . . .16 mm. 



From side to side . . . .13 mm. 



Localities. — England : the Carboniferous Limestone series of Lowick and the 

 Fourlaws Limestone at the Coomb, near Redesdale, Northumberland. Ireland : 

 Killycloghy, Lisbellaw, co. Tyrone. 



Observations. — The type of SoAiguinolites costellatus, M'Coy, consisting of a 

 fragment of a cast of the posterior end of the left valve, is preserved in the 

 Griffith collection in the Museum of Science and Art, Dublin. It is a poor 

 example, but shows faint indications of the radiating ribs. M'Coy's description 

 is as follows : " transversely oblong, depressed, anterior side short, rounded, 

 marked towards the margin with four or five short, flat, radiating ribs." 



8. radiata was established by Meek and Worthen for a shell from the Coal- 

 measures of Illinois, U.S.A., and I have no hesitation in referring the American 



