416 Systematic Paleontology 



duced, are a little flattened. The mesial sinus is very shallow, being often 

 little more than a flattening of the surface along that part of the shell. 

 The umbo is broad, and the beak a little incurved over the area. Area in 

 young shells narrow, and in old shells proportionally wider, and extending 

 to the cardinal extremities : the foramen is wide, and, in old shells, par- 

 tially closed above (by deltidial plates). The dental lamellae are strong; 

 the extremities, rising above the cardinal line and bending backwards be- 

 neath the area, are widely divergent as they extend from the beak down- 

 wards. The muscular impressions are large, and very strongly and 

 beautifully marked. Dorsal valve of the same form as the ventral; its 

 greatest convexity in the middle, with a verj- narrow cardinal area, above 

 which the beak is slightly incurved. Mesial elevation very moderate, and 

 sometimes scarcely defined. 



" Surface marked by reg-ular simple rounded or sometimes subangular 

 plications, of which there are from ten to twenty on each side of the mesial 

 fold and sinus. The mesial sinus is simple at the apex; but a plication 

 becomes developed in the bottom of it, and each of the bordering plications 

 is dichotomized ; the central one dichotomizing once, and in old shells 

 twice, before reaching the margin. Very young shells show a sinus with a 

 simple plication in the bottom. The mesial elevation of the dorsal valve 

 is simple in very young shells, showing first a central groove, then each 

 marginal plication becomes dichotomized, and at the same time a central 

 plication rises in the median groove; and the mesial fold, at its base, 

 consists of five [or six] distinct plications, the result of the dichotomizing 

 of a single one at the apex. Surface marked by fine concentric striae and 

 stronger imbricating lines of growth. [These are traversed by very deli- 

 cate radial interrupted striae.] In the casts of the interior, this fossil 

 presents considerable variety of appearance, owing to the variable extent 

 of the muscular area, the development of its markings, and depth of the 

 cavity beneath the beak; characters due in part to the different ages of 

 the shell, but often apparently to other causes." Hall, 1859. 



This well-known and diagnostic fossil of the Oriskany is the most com- 

 mon shell about Cumberland. It is distinguished from all the associated 

 spirifers by the plicated fold and sinus. In Washington County this 



