STROPHOMENIDtE. 287 



following the curves of the opposite one ; hinge-area narrow, with an arched pseudo- 

 deltidium corresponding to that in the ventral valve. External surface of both valves 

 covered with numerous radiating, coarse, thread-like radii, of almost equal width in all 

 their length, and separated from each other by interspaces five or six times as wide ; 

 the interspace being filled up with from five to eight very fine, hair-like, longitudinal 

 striae, and an occasional slightly larger riblet occupying the centre of the interspace, 

 so that the principal ribs increase in number by interpolation at variable distances from 

 the beaks. In the interior of the dorsal valve a bifid cardinal process, grooved along the 

 middle, is situated between two small laminar processes, on the outer side of which is 

 the hinge-socket. A narrow slightly projecting ridge extends along the middle of the 

 valve to about its centre, and on either side are two parallel muscular (adductor) im- 

 pressions, ramified and oval, those situated close to the central ridge being the smallest, 

 and often the most prominent. Interior of the ventral valve is not known. Two 

 specimens measured — 



Length 25, width 29, depth 2^ lines. 

 •'>4 24 2 



Obs. Pander, who first named and described this species, had in view a shell with 

 geniculated valves, and I am still uncertain whether we are correct in uniting to it the 

 very gibbous or convex form which we find principally in the Upper Silurian formation. 

 It is true M. de Verneuil has informed us of having found, in the same Lower Silurian 

 rock which contains the typical form of Pander's species a specimen regularly convex 

 (' Geol. of Russia,' pi. xv, fig. 3"), and thus, as it were, connecting the geniculated with 

 the convex variety. In our British Silurian rocks we have not yet discovered the typical 

 geniculated form ; and, as all our specimens are regularly convex, I think it will be 

 advisable to distinguish this .modification by a special designation. I therefore 

 propose for the convex form the varietal name of semiglohosa. As will be seen from 

 the list of references above recorded, the convex form has been hitherto constantly 

 referred to Strophomena imbrex. 



In the Caradoc or Bala Limestone of Craig Head Quarry, near Girvan, in Ayrshire, 

 a smaller convex variety, much resembling our Upper Silurian form, has been found 

 (figs. 5 and 6) ; but it presents the peculiarity of possessing a small circular foramen 

 at the extremity of the beak of its ventral valve. Unfortunately, we do not know its 

 interior ; and, as the external shape and striation so exactly agree with that of the 

 var. semiglohom, we have left it provisionally with it. StropJiomena imbrex, var. semiglobosu, 

 is also apparently specifically distinct from Strophomena Ouralinensis, chiefly by its 

 external sculpture ; and I was informed by Mr. Salter that the specimen so named 

 by Prof. M'Coy, and now in the Woodwardian Museum, is referable to the shell under 

 description. It is from the Woolhope Limestone of Presteign, and is not the Leptana 

 Ouralinensis of De Verneuil. I know, however, little about this last-named shell, further 

 than what is said of it by M. de Verneuil in the work on the ' Geology of Russia,' &c. 



