Maryland Geological Survey 405 



and incurved. Fold distinctly defined by a sharp depressed line on either 

 side, but not elevated much above the general surface of the valve. It 

 gradually and uniformly widens forward, is broadly rounded on top, and 

 is occasionally marked by a slight central depression. Ribs almost obso- 

 lete, a faint depression outlining the first on either side of the fold in some 

 specimens. Surfaces of both valves marked by fine, uniform, and sub- 

 equally spaced concentric lines which curve forward in the sinus of the 

 pedicle valve. Occasionally strong lines mark a temporary resting stage 

 during growth. The whole surface appears to be covered with fine radiat- 

 ing stris, which are interrupted by the concentric stri;E, thus giving the 

 surface a fimbriate appearance. On the interior of the pedicle valve are 

 two short dental plates, diverging slightly more than the sides of the sinus. 

 The cardinal area of this species is high, occupying in some specimens as 

 much as a third of the total height of the valve. The strength of the ribs 

 on the brachial valve varies somewhat in different specimens, but they 

 are always much less marked than those of the pedicle valve, and they 

 are usually quite obsolete. 



" The species to which this most nearly approaches is the variety of S. 

 crispiis Hisinger found in the Coralline limestone at Schoharie. In this 

 variety the ribs are much fainter than in the normal S. crisjms of the 

 Niagara shales and limestones of western New York. In many specimens 

 from Schoharie the ribs are almost obsolete, comparing well with their char- 

 acter in S. erie?isis. The sinus of the Schoharie specimens is subangular, 

 and the fold flattened much as in the Bullhead limestone species. This 

 variety is also proportionally higher than the normal form, giving a sub- 

 rhomboidal outline to the pedicle valve, which strongly recalls S. eriensis. 

 In general the ribs of this latter species are slightly broader and rather 

 more flattened on top than is the case in the Coralline limestone species, 

 and the interspaces are somewhat narrower. Taking all the variations 

 into consideration, a very close relation must be accepted as existing be- 

 tween the two species. The specimens described by Whitfield as S. 

 vanuxemi from the hydraulic limestone (Manlius?) of Peach Point, Put- 

 in-Bay, Lake Erie, resembles rather more closely the normal S. crispus 

 than it does the typical S. vanuxemi of the Manlius limestone of central 



