EDMONDIA COMPRESSA. 305 



The umbones are of moderate size, elongate, tumid, twisted inwards and for- 

 wards, only slightly raised above the hinge-line and situated very far forwards, 

 and excavated in front. The valves are evenly and gently convex, except in front, 

 where there is a rapid compression. The dorsal slope is very gentle, and the pos- 

 terior end is narrowed by the approach of the dorsal and ventral border. 



Interior. — Few details have been yet observed. The interior of casts is almost 

 smooth. The hinge-plate is furnished with the internal, elongated, expanded 

 ossicle which obtains in other species of the genus. 



Exterior. — The surface is covered with numerous very fine regularly arranged 

 concentric lines of growth. Shell thin. 



Dimensions. — PI. XXXIII, fig. 7, M'Coy's type specimen, measures — 

 Antero-posteriorly . . . .50 mm. 



Dorso-ventrally . . . .33 mm. 



Elevation of valve . . . .6 mm. 



Localities. — England : the Carboniferous Limestone of Thorpe Cloud and Castle- 

 ton, Derbyshire ; Carboniferous Limestone, Isle of Man (M'Coy). Ireland: Cork. 



Observations. — E. compressa differs from E. union if or mis, being more trans- 

 verse, less gibbose, and more oblique, having the umbones more anterior and 

 more prosogyrous, and the surface-markings much finer than in the latter species. 



E. compressa was described twice by M'Coy, op. supra cit. The type which served 

 for description is preserved in the Griffith Collection of the Science and Art Museum, 

 Dublin ; and I am fortunately able to refigure this specimen, PL XXXIII, fig. 7. 

 The second description was evidently based on a specimen from the Carboniferous 

 Limestone of the Isle of Man, but was accompanied by no figure. In the later 

 description, a much fuller one than the former, the dorsal margin is said to be 

 nearly straight, but in the original account it was described as " obtusely rounded." 

 The latter is correct, for although the type specimen is incomplete posteriorly, 

 observation shows that the lines of growth representing the contour of the shell 

 in a young state are distinctly rounded. Allusion is also made to a lunette, but no 

 lunule occurs in the genus Edmondia. 



E. compressa more closely resembles E. transversa than any other species of 

 the genus. The latter is more convex; its upper and lower margins are sub- 

 parallel and the posterior end is truncate, and even in casts have well-marked con- 

 centric ridges and sulci. The umbones are somewhat less anterior, and the shell 

 only attains to about half the size of E. compressa. 



Edmondia (?) prselonga, de Koninck, has somewhat the general character of E. 

 compressa, but it has its dorso-ventral diameter much shorter ; but this is the case 

 in young specimens of the latter species, as may be noted by observing some of the 

 earlier lines of growth in the type specimen. 



