Gryphaea
Species
Gryphaea impressimarginata McLearn, 1924
Differs from G. nebrascensis in being more robust and having coarse ribbing on umbo. Elongate to subtrigonal outlien, with convexity ranging from moderate to strong.
Occurrence: Sawtooth (common at top, uncommon in base), Rierdon (base), Twin Creek (lower)
Reference: Imlay, 1948
Gryphaea impressimarginata
Photograph from Imlay, 1948
Gryphaea nebrascensis Meek and Hayden, 1861
Elongate shale, prominent and narrow umbo, strongly incurved beak, with radiating striae on umbo
Occurrence: Extraordinarily abundant in the Stockade Beaver Shale, ranges to top of marine Jurassic; “lower” Sundance (oolitic limestone, granular limestone, and claystone) at Lower Slide Lake; Leeds Creek Member of Twin Creek
Reference: Imlay, 1948; Imlay, 1967
Gryphaea nebrascensis
Photograph from Imlay, 1948
Gryphaea planoconvexa Whitfield, 1876
Subcircular, with moderately inflated left valve
Occurrence: Sawtooth (middle), Piper (middle), Gypsum Spring (middle), and Twin Creek (basal); middle and upper Rich Member of Twin Creek Limestone
Reference: Imlay, 1948; Imlay, 1967
Gryphaea planoconvexa
Photograph from Imlay, 1967
Gryphaea planoconvexa fraterna Imlay, 1967
Differs from G. planoconvexa by having a “much more convex left valve, a subovate to subtrigonal instead of subcircular outline, a higher more strongly incurved umbo, and a posteriorly directed beak, and by weak radial riblets on some specimens.”
Occurrence: Sliderock and Rich Members of Twin Creek Limestone
Reference: Imlay, 1967
Gryphaea planoconvexa fraterna
Photograph from Imlay, 1967
