SPIEIFEK^ OF THE HAMILTON GROUP. 227 



The figures 10-16 illustrate the prevailing form and characters of this species. 



Fig. 17 is of the interior of the dorsal valve : the interior of the ventral valve is unknown. 



Fig. 18 is an enlargement of the surface. 



Geological formation and localities. The original specimens described were 

 from the Hamilton group at Covington in Genesee county. It has since been found 

 among collections from the shores of Ca3aiga and Seneca lakes; at Dresden in 

 Yates county; on the shore of Cauandaigua lake, and at Geneseo and York in 

 Livingston county. 



Spirifera niedialis. 



PLATE XXXVIIL 



Delthyru mtdialit : Hail, Gcol. Report Fourth District New-York, p. 208, f. 8. 1813. 



Spiri/er medialis : Uall, Tenth Report on State Cabinet, p. 164. 



CotLpare Detthyrit oudocu/a, Cokbad, Jour. Acad. Nat. Sciences, Philadelphia, Vol.viii, p. 262. 



Shell subtriangular, semicircular or subelliptical : valves moderately 

 convex in young specimens, becoming ventricose in old shells ; hinge- 

 line usually extended beyond the width of the shell below, sometimes 

 mucronate at the extremities : surface plicate. 



Ventral valve usually deeper than the dorsal : beak prominent, more 

 or less incurved. Area generally of more than medium height, longi- 

 tudinally striate and divided in the middle by the deltoid fissure, which 

 is twice as high as wide, and reaches to near the apex of the valve. 

 Mesial sinus of moderate width, reaching to the apex, rather deep, 

 generally rounded at the bottom, but sometimes a little flattened ; the 

 margins subangular and distinct. 



Dorsal valve moderately convex or gibbous, the greatest convexity 

 above the middle : beak small and slightly incurved. Area linear and 

 well defined ; the lateral extremities of the valve a little deflected 

 along the hinge-line. Mesial fold prominent, rising abruptly at the 

 sides, rounded or slightly flattened on the top. 



Surface marked by fcom twenty to thirty simple plications on each side 

 of the mesial fold and sinus. These are of medium size in full-grown 

 shells, being either low rounded, flattened, sharply rounded, or sub- 

 angular in different individuals ; the principal ones are frequently 

 marked along the centre by a fine threadlike groove, a feature most 





