108 INTRODUCTION. 



value of the character to be derived from the pattern of the vascular impressions is con- 

 siderably reduced, from the positive fact of a species, S. expansa, Sow.,^ presenting along 

 with the hinge and general habit of S. alternata, an arrangement of the vessels more like 

 L. depressa or analoga, which, with a slight modification, are reflected, as in the last-named 

 species, to surround the ovarian spaces, &c. (PI. VIII, fig. 162.) Mr. Salter and myself 

 were of opinion that we might, perhaps, in the present state of our knowledge, divide 

 the genus Strophomena into three sub-sections, thus : — 



Sub-section 1. S. alternata, plajiumhona, groMdis,filosa, euglypha,funiculata, &c. 



2. S. expansa (the only species with certainty known.) 



3. 8. depressa, analoga, Bouei, &c. {Leptagonia, M'Coy). 



But I think it will be preferable not to introduce these divisions until more is known of 

 the pattern of the vascular impressions and other characters of the hitherto unexamined 

 species, as well as of their general habit. The area in some forms, such as S. latissima, 

 Bouch. ; Naranjoana, Vern., &c., has no deltidium or fissure, and yet the internal characters 

 agree with other Strophomence. 



Geol. range. — This genus contains some of the oldest forms of animal life, abounding 

 in the lower and upper Silurian, Devonian, and Carboniferous periods, but it is not 

 positively known above that epoch. 



Examples : with the socket valve concave ; L. alternata, depressa, analoga, imbrex, 

 Loveni ; &c. 

 with the socket valve convex; L. euglgpha ; funiculata, antiquata, plano- 

 convexa, planumhona, pecten, sulcata, &c. 



Genus^—ljEYiMYix, Batman, 1827, restricted. 



Type — L. transveksalis, Dal. Int. PI. VIII, figs. 176 — 185. 



Anomia, Linnceus (part). 



Lept^na (part), of Dalman, 1827, as well as of the generality of Authors. 



Plectambonites, Pander, 1830. 



Animal unknown ; shell involute, semicircular, transverse or elongated ; surface 

 smooth or variously striated ; dental or ventral valve (always ?) regularly convex ; socket 

 valve concave and following the curve of the other ; area double, fissure in the dental valve 

 partly covered by a deltidium ; summit sometimes perforated by a small circular aperture ; 

 valves articulating by means of teeth and sockets ; in the interior of the dorsal valve the 

 socket ridges are largely developed, the cardinal process or boss small, multifid, and 



' Mr. Salter called my attention to this remarkable species, figured by himself in the ' Silurian 

 System,' pi. xx, fig. 14, and having borrowed the original specimen now in the Mus. of the Geol. Society, 

 and compared it with other examples in the Mus. of Practical Geology, I was enabled to restore the entire 

 impressions, incomplete from fracture in the original examjjle. 



