Wednesday, June 20, 2012

Pterinea brisa Pelecypod

This fossil imprint was a nice find. It appears to be a Pterinea brisa Hall (or Ambonychia (Pterinea) striaecostata McChesney 1869) pelecypod fossil found in the Silurian Waldron Shale of Clark County, Indiana. A similar bivalve is found in the Rochester Shale of New York called Cornellites emacerata Conrad.

The Pterinea brisa was described by James Hall in the 1881 Indiana Geological Survey (pages 316-317; Plate 27 Figures 24-25; Plate 28 Figures 7-9):
Body of shell (left valve) obliquely subovate, extremely inequilateral; anterior wing moderately extended and strongly sinuate at its junction with the body; posterior wing not extending as far as the posterior extremity of the shell; umbo prominent, beak rising a little above the hinge-line; muscular
impression in right valve large and nearly round, near the middle of the length of' the shell. In the casts, beneath or just anterior to the beak, there is one short curving dental pit, with. smaller accessory one. This is a feature not observed in the Waldron specimens, which are usually found as the exteriors and often as. imperfect interiors of the valves; the few casts which have been observed from this locality, do not show the muscular marking. 
Surface marked by strong radiating and concentric striae, with broad, little elevated, radiating ribs. The concentric striae of growth are produced into prominent, recurving, fimoriated laminae, the fimbriae being infolded at their margins and bending backward in the form of long, hollow, semicylindrical spines, leaving a broad sinus between, in the bottom of which is a shorter spine. The younger specimens, as usually preserved, present simple fimbriated lamellae.

Plate 28 Figure 7 - Indiana Geological Survey 1881

Plate 28 Figure 8 - Indiana Geological Survey 1881