OEISKANY FAUNA OF BEORAFT MOUNTAIN 57 



formity. The great convexity and abrupt slope of Brach. ma j lis 

 are absent. Along the middle the brachial valve is depressed by a 

 longitudinal sinus widening anteriorly, seldom well defined at its edges. 

 This is well marked on the interior of the valves and shows itself 

 on the exterior of the pedicle-valve as a broad median ridge passing 

 without definition into the lateral slopes. Hinge-line straight and as 

 long as the width of the shell. Umbo of the pedicle-valve convex 

 and prominent. 



The surface of early growth stages is marked by a few coarse, 

 strong, subangular ribs with a few secondary striae. These rapidly 

 increase by division and implantation, so that eventually but , little 

 trace of fasciculate arrangement is left, and the surface becomes 

 equally, coarsely and closely striated. These striae are crossed 

 by fine concentric lines, which are sharper than those in Brach. 

 ma jus, and present a different aspect, on account of the absence of 

 the finer radiating striae which they cancellate in that sjiecies. Cardinal 

 and internal characters as in Brach. m a j u s , the row of cardinal 

 denticulations being somewhat larger than in that species. 



Early stages of this species are, as observed, suggestive of 

 Brach. aratum Hall, and in some cases recall specimens of S t r o  

 pheodonta demissa Conrad, from the Hamilton shales. 



Leptaena rhomboidalis Wilckens 



1892. Leptaena rhomboidalis Clarke, op. cit. p. 413 



The shells of this species are common and conform to the type 

 of this shell occurring in the Helderbergian and Ulsterian rather than 

 to the large and ventricose form of the Oriskany which has been 

 described by Hall as Strop ho mena rugosa, var. ventricosa, 



Dalmanella perelegans Hall 



1859. Orthis perelegans Hall, Paleontology of New York. 3:171, pi. 13, 



fig. 4-12 

 1892. Orthis perelegans Clarke, op. cit. p. 413 



Shells which can be ascribed to this species have usually under- 

 gone some compression which obscures in some degree their specific 



