SPIKIFER^ OF THE UPPER HELDERBERG GROUP. 189 



SPIEIFERiE OF THE UPPER HELDERBERG GROUP. 



^pii'ifcr (luodenaria. 



PLATE XXVII & XXVIII. 



Delthyris duodenaria : Hall, Geol. Report 4th District New-York. p. 171, f. 5. 1843. 

 Spiriftr duodenaria ; Hall, Catalogue in Report on State Cabinet. 

 Spiri/era duodenaria. cited by Billings, Canadian Journal, p. 256. 1861. ' 



" " Geology of Canada, p. 372. 1803. 



Shell transverse, semicircular : hinge-line equalling the greatest width 

 of the shell ; cardinal extremities obtuse or acute, rarely acuminate. 

 Valves subequally convex ; area very narrow. Surface plicated. 



Ventral valve moderately gibbous, arcuate, compressed towards the 

 cardinal extremities. Mesial sinus of moderate width and depth, 

 rounded or slightly flattened on the bottom ; umbo prominent, the 

 beak small, neatly curved over a wide triangular fissure, and reach- 

 ing to within half a line of the umbo of the opposite valve ; area con- 

 cave, sublinear, a little wider on each side near the centre. 



Dorsal valve regularly convex, a little gibbous in the middle, and flat- 

 " tened or sometimes slightly concave at the cardinal extremities. Mesial 

 fold rather narrow, rounded, prominent and strongly defined, some- 

 times a little flattened on the middle. The surface is marked by six 

 and rarely seven strongly rounded ribs on each side of the mesial fold 

 and sinus. The ribs gradually decrease in size and prominence from 

 the centre, and the outer ones are often scarcely elevated in young or 

 medium-sized individuals. 



The entire surface is marked by lamellose concentric striae, giving a 

 papillose or subfimbriate aspect at their junction. It usually happens, 

 however, that the surface is smooth from jiartial exfoliation. 

 In the Schoharie grit, the cast of the ventral Valve shows a somewhat 



narrow muscular area, with the sides subparallel for half their length 



and contracting below. There are faint indications of a median crest. 

 In the interior of a ventral valve from the Corniferous limestone, 



Plate 28, fig. 23, the muscular area is broad and rounded, with a distinct 



