EDMONDIA ARCUATA. 311 



Edmonbia arcttata, King, 1849. Monogr. Permian Foss., p. 104. 

 Sanguinolites arcuatus, Broivn, 1849. Ulustr. Foss. Conch., p. 219, pi. xc, 



fig. 10. 

 Ltonsia arcuata, d'Orhigny, 1850. Prodrome de Paleontol., p. 128. 

 Sanguinolites arcuatus, Morris, 1854. Cat. Brit. Foss., 2nd edit., p. 223. 

 Edmondia arcuata, Bigshj, 1878. Thesaurus Devonico-Carboniferus, p. 307. 



— — Lebour, 1878. Outlines Geol. Northumberland and Durham, 



p. 121. 

 — Etheridge, 1888. Brit. Foss., pt. 1, Palaeozoic, p. 283. 



Specific Characters. — Transversely elongate, elliptically almond-shaped, very 

 inequilateral, gibbose, slightly oblique. The anterior end is very short, depressed, 

 compressed much below the level of the umbones, and has its border erect, almost 

 semicircular in curvature, passing into the inferior margin, which is elongate, and 

 almost straight for the greater part of its extent, but is curved upwards at the 

 posterior end. The posterior margin is bluntly but regularly rounded. The 

 hinge-line is long and gently arched. The umbones are small, elongate, incurved, 

 contiguous, not much raised above the body of the shell, and placed in the anterior 

 fifth of the valve, much excavated anteriorly. The valves are regularly convex 

 from before backwards and above downwards, so much hollowed out anterior to the 

 umbones as to be concave. The dorsal slope is broad but only slightly depressed, 

 and in old specimens there is the slightest approach to obtuseness along a line 

 passing from the umbo to the postero-inferior angle. Owing to the narrow 

 anterior end and the obliquity of the valve, the posterior end has a false appear- 

 ance of being somewhat expanded from above downwards. 



Interior. — The anterior adductor muscle-scar is large, triangularly ovate, and 

 situated remote from the margin in the hollow at the base of the anterior limb of 

 the umbo, bounded behind by a shallow groove. Immediately above this, and in 

 the hollow of the anterior limb of the umbo, are some deep accessory muscle- 

 scars. The posterior adductor is large, shallow, and placed close to the postero- 

 superior angle. In casts, external to the hinge-plate, is an elongate narrow groove, 

 expanding and becoming shallower posteriorly, in which lodged the ossicle peculiar 

 to the genus. In well-preserved casts there is also, posterior to the umbones, an 

 elongate depression, marked off by a curved line, which starts immediately behind 

 the umbo, and, turning outwards at first, returns to the edge of the ridge, which 

 is above the slit for the ossicle and probably represents a thickening of the roof of 

 the umbo for the support of this process. Hinge edentulous. The interior of 

 the shell has markedly shallow concentric grooves and ridges. Pallial line entire. 



Exterior. — The surface of the valve is covered by numerous, distinct, fine con- 

 centric lines of growth, with here and there a deeper sulcus, all of which com- 

 mence and end in the upper margin of the shell, curving completely round each 

 end. Shell very thin. 



