STROPHOMENID^. 327 



Spec. Char. Variable in shape ; rotiindato-quadrate or semicircular ; either shorter 

 or longer than wide ; sides sometimes almost subparallel ; front rounded. Hinge-line 

 nearly or quite as wide as the shell ; cardinal extremities obtusely angular, or 

 presenting short semicylindrical or convoluted ears. Ventral valve either evenly convex 

 and flattened at the wings (fig. 7), or longitudinally flattened and slightly depressed 

 along the middle (fig. 12). Area triangular, rarely exceeding one line in height ; fissure 

 triangular, covered by a pseudo-deltidium ; beak small, incurved. Dorsal valve concave, 

 following the curves of the opposite valve ; hinge-area almost linear. Surface of both 

 valves crossed by numerous, close, fine, thread-like, concentric lines or wrinkles. In the 

 interior of the dorsal valve, on either side of the fissure, is placed a small prominent 

 hinge-tooth. The muscular scars form two, long, narrow, deviating depressions, with 

 raised margin (convex in the cast at fig. 18), and separated by a wide interspace. 

 These scars extend almost to the frontal margin ; and are in all probability due to the 

 divaricator muscle ; while posteriorly and at a short distance only from the beak are two 

 smaller cavities (convexities in the cast), which may be referable to the adductor muscle. 

 In the interior of the dorsal valve are two widely deviating, narrow, brachial (?) laminae, 

 on the outer side of which are situated the hina-e-sockets and hinge-area. The adductor 

 muscular depressions are in the shape of two elongated, subparallel, oval scars, which 

 extend to near the frontal margin, and are separated from each other by an interspace 

 about equal in width to one of the scars ; while a narrow prominent ridge runs along the 

 middle of the valve. The muscular depressions are also posteriorly divided by a trans- 

 verse ridge ; and a prominent raised border surrounds the whole muscular space (figs. 

 16, 17). The internal surface of the ventral valve (not occupied by the muscular 

 depressions) is also very rugose (fig. 18). Two specimens measured — 

 Length 9, width 10, depth 1 line. 



Obs. In 1846 Prof. M'Coy described and figured an incomplete ventral valve (a cast) 

 of this shell under the designation of Producta tenuicincta, mistaking the rugosities for 

 " a few scattered, lengthened spine-bases." I saw the original specimen in the collection 

 of Sir R. Grifiith, and was able to ascertain that it belonged to a Leptcena ; and for this 

 the original designation of teimicijicta w\\\ require to be retained. Prof. M'Coy (in 1846) 

 did not omit to observe and figure the fine close concentric striae with which the surfaces 

 of the valves (when perfect) are ornamented. I am consequently at a loss to understand 

 why, while describing his Leptana tenuicincta, at page 239 of the work on ' British 

 Palaeozoic Fossils,' he observes, " This beautiful species seems almost identical in size, 

 shape, and marking, with the Producta tenuicincta (M'Coy) and Leptcena (enigma (Vern.), 

 but has a very wide, distinctly marked, cardinal area " — which, I would add, cannot be 

 seen in the original so-termed Producta, this part of the specimen being concealed by the 

 matrix. 



The shell is very variable in shape, as may be seen from the many examples figured 



