No. 109.] Ill 



Strophomena ( Strophodonta) crenistria. 



Pal. N.Y. Vol. iv. 



Strophomena crenistria, Hall : Rep. Gcol. 4th Dist. New-York, pa. 171. 



pi. 35, f. 3 &. 4. 



Shell somewhat semicircular, much cojnpressed ; length from 

 two-thirds to three-fourths as great as breadth ; hinge crenulated, 

 nearly or quite equal to the greatest width of the shell, and 

 sometimes having small salient angles at the extremities : dorsal 

 valve unknown : ventral valve much compressed. Surface 

 marked by fine strise, increasing by bifurcation and interstitial 

 addition ; crossed by fine closely set concentric lines of growth 

 and a few indistinct wrinkles, giving a subcrenulate aspect to 

 the radiating stride : interior granulose ; granules on each side 

 of the fan-shaped vascular impression, larger than those nearer 

 the margins. 

 This shell has much the form and general aspect of Lepttsna phiristriata 



( Conrad ) from the Hamilton group, but has rather coarser and more 



rounded striae, which are not so sharply crenulated. 



Geological position and locality. Limestone of the Upper Helderberg 



group, Williamsville, N.Y. 



StEOPUOMENA CRENI9TEIA. 



Strophomena ( Strophodonta) ampla. 



Pal. N.Y. Vol. iv. 

 Shell large, transverse, nearly semicircular; hinge-line crenulated, 

 equal to the greatest width of the shell : dorsal valve prominent 



