STROPHOMENIDtE. 313 



sides and front rounded. Ventral valve very slightly convex ; area narrow, eighteen to 

 twenty times wider than high; fissure covered with a narrow pseudo-deltidium. Dorsal 

 valve very slightly concave, or almost flat. Surface of valves covered with numerous, 

 rounded, raised, thread-like radii, with interspaces of greater width, in which are one 

 or two smaller ribs, commencing at various distances from the beaks, and extending to 

 the margin ; the whole surface is likewise crossed by numerous, close, equidistant, 

 concentric ridges, as well as by a few deeper lines of growth. In the interior of the 

 ventral valve is a prominent tooth on each side of the fissure, supported by small dental 

 plates, which enclose a very large, saucer-shaped, muscular cavity or depression, divided 

 along the middle, and radiated by numerous ridges and furrows. The vascular impres- 

 sions consist of two primary trunks, which run parallel direct from the anterior portion 

 of the muscular depressions to the frontal margin, and give off" laterally, at intervals, 

 all along their extent, horizontal bifurcating branches towards the lateral margins of the 

 valve (fig. 6) ; the first or posterior branch on each side is reflected round the ovarian 

 space, which is contiguous to the muscular cavity. In the interior of the dorsal valve 

 the cardinal process is bifid, each lobe being grooved or excavated along its middle ; on 

 the outside is situated the dental socket, surrounded by a raised elevation. Under 

 the cardinal process is a small median ridge, which separates the adductor muscular 

 impressions into two pairs, each pair being again divided by a flattened ridge (fig. 10). 

 Length 26, width 30, depth 2 lines. 

 Obs. I cannot help expressing some surprise that so able an observer as is Prof. 

 M'Coy should not at once have perceived that the shell under description was not an 

 OrtJds, but a Strojohomena, the concavo-convex aspect of the shell, the fissure closed by 

 a pseudo-deltidium, the bifid cardinal process, the position and shape of the muscular 

 scars, all denoting that Sowerby's species could not possibly be an Orthis ; and still more 

 so, as further on he rightly recognises the genus of the closely allied form Stropliomena 

 compressa. Strophomena expansa attained large dimensions, relatively to other species of 

 this genus in our British Silurian rocks. Sowerby figured the exterior of one of its valves 

 under the designation of Orthis pecten,—^ species not positively known to occur so low 

 down in the series, and differing from the former very materially in the shape of its 

 muscular and other internal impressions. Orthis expansa, var. concentrica, Portlock, is 

 also another form of this variable species. Strophomena expansa is described at consider- 

 able length by Mr. Salter at p. 378 of the second volume of the 'Memoirs of the Geol. 

 Survey,' from which I will transcribe some passages. "' The surface along the hinge-line 

 is sometimes corrugated in oblique folds ; and, I believe, in this state [this species] is the 

 Orthis rugifera, Portlock ; the raised threads, interlined by three or five fine ones, of 

 which the central one afterwards becomes a strong thread, are arched upwards towards the 

 sides, not radiated in straight lines ; and the central triangular portion at the beak, 

 marked off by the two diverging hinge-plates, is raised up a little on the exterior. The 

 most remarkable character resides in the muscular impressions, which are subrhomboidal, 



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