STROPHOMENID^. 303 



Strophomena Hollii, Dav. PI. XLII, figs. 18, 19. 



Spec. Char. Shell minute, wider than long, greatest breadth at the long straight 

 hinge-line ; cardinal extremities rectangular, lateral margins rounded, slightly indented in 

 front. Ventral valve moderately convex, with a shallow longitudinal depression or sulcus 

 along the middle ; beak not produced, perforated with a minute circular foramen near to its 

 extremity; area narrow; fissure wide. Dorsal valve not known. In the interior of ventral 

 valve there is a prominent tooth on each side of the fissure ; the saucer-shaped muscular 

 cavity is deep, with a raised margin, and divided longitudinally by a prominent ridge, 

 which extends along the middle ; the small circular foramen lies under the extremity of 

 the beak and close to the posterior edge of the muscular cavity. The remainder of the 

 interior of the valve is divided into two deep cavities by a slightly raised mesial ridge. 

 Length 1, width 1^ lines. 



Obs. This little species occurs by thousands in a yellow Caradoc Shale at the Onny 

 section ; where it was first discovered by Mr. Lightbody, who forwarded me specimens. 

 It is unfortunately very minute, and is found as internal casts. I could only make out 

 that belonging to the ventral valve. Some traces or impressions of parts of the external 

 surface have also been noticed, but the sculpture was very obscurely defined, and consisted 

 of numerous fine strige. No doubt, with diligent search in the locality, examples of the 

 dorsal valve, and others exhibiting the external surface, will be discovered. In the mean 

 time we can offer no more than a very incomplete description. 



Strophomena Siluriana, Dav. PL XLVII, figs. 1 — 4. 



Spec. Char. Shell thin, transversely semicircular, gently concavo-convex ; hinge-line 

 as long as, or a little less than the width of the shell ; cardinal extremities angular, some- 

 times slightly protruding in the shape of raucronate wings ; sides and front rounded. 

 Ventral valve gently and uniformly convex ; beak slightly incurved, but not projecting ; 

 area narrow, about ten times wider than high ; fissure moderately wide, triangular, and 

 partly arched over by a small pseudo-deltidium. Dorsal valve gently concave, following 

 the curves of the opposite valve ; hinge-area very narrow. External surface marked by 

 numerous unequal, radiating, fasciculated, raised striae. The ribs on leaving the beaks are 

 few in number and strong, with wide interspaces between them ; each primary rib 

 extends straight to the margin and remains the most prominent ridge in each cluster. 

 At a little distance from the beaks most of the ribs commence laterally to bifurcate into 

 two or three ; and, as these approach the margin, they split again into numerous smaller 

 divisions, so that close to the margin each of the primary ribs has become, during its 



