STROPHOMENIDiE. 315 



Professor's work on ' British Palseozoic Fossils,' some uncertainty existing in my mind as 

 to this species having really been obtained from some of the localities there quoted. 



Strophomena compress a, Sow. (sp.). PI. XL VI, figs. 7 — 10. 



Orthis compeessa, J. de C. Sow. Sil. Syst, pi. xxii, fig. 12, 1839. 



Strophomena compressa, Phillips and Salter. Mem. Geol. Surv., vol. ii, p. 379, 



1848. 

 LEPTiENA — M'Coy. Brit. Pal. Foss,, p. 242, 1852. 



— — D^ Eichwald. Letheea Rossica, vol. i, p. 873, 1859. 



Strophomena — Salter. Siluria, 3rd ed., p. 100, fig. 7, and pi. ix, fig. 



16, 1859. 



Spec. Char. Semicircular or squarish, wider than long; hinge-line either slightly 

 exceeding the width, with short rounded ears ; or rather less than the width, with obtusely 

 angular extremities ; sides and front margin rounded ; ventral valve gently convex ; area 

 narrow; fissure small, arched over by a pseudo-deltidiura. Dorsal valve nearly flat, or 

 rather very gently concave ; hinge-area narrow. External surface of both valves marked 

 with numerous, fine, thread-like, slightly projecting radii, with one or two riblets between 

 each pair of the principal radii, these last becoming larger and closer in their turn, with 

 punctures along the intervening furrows. The whole surface is closely imbricated by 

 equidistant, concentric, prominent ridges. In the interior of the ventral valve the 

 muscular scars are large, and form two wide lobes, separated in the middle by a flattened 

 ridge. In the interior of the dorsal valve the cardinal process is bifid, and placed between 

 two small deviating plates ; the muscular scars below being separated by a median 

 ridge. 



Length 14, width 19, depth \\ lines. 

 Obs. As was justly observed by Mr. Salter, it must be confessed that in general form, 

 size, and convexity, this shell closely approaches the last, but the striation is far more 

 irregular. I may also point to the great general resemblance of the internal details ; and 

 I should not be at all surprised if it were no more than the Llandovery descendant of the 

 Caradoc Strophomena expansa. It is, however, a smaller shell ; and, as Palaeontologists 

 generally have considered it distinct, I shall retain it so provisionally. Indeed, Prof. 

 M'Coy appears to feel quite sure that it is so, for he states, at p. 242 of his work on 

 ' British Palaeozoic Possils,' " The great compression of the valves, thinness of the shell, 

 the marked concentric waves of growth, thread-hke subequal striae, and large punctures, 

 easily distinguish this in wellmarked specimens from Orthis expansa or 0. alternata, as 

 well as the difi'erence in the cardinal area, and foramen, and muscular impressions, and 

 simple rostral tooth." (I question the accuracy of these last so-termed differences ; the 



