ORISKANY FAUNA OF BECK AFT MOUNTAIN 55 



bergian Stropheodonta varistriata (Conrad) Hall. While both 

 complete all the stages of growth represented by that species, they 

 alike pass beyond those and attain adult characters not exemplified in 

 species of earlier date. Brachyprion varistriatum is a com- 

 mon species in the Manlius limestone, and, though usually of small size 

 in the former, sometimes assumes considerable dimensions. Prof. Hall 

 has described as a variety of this species, Stroph. varistriata, 

 var. a rat a, from "a crystalline band of the shaly limestone of the 

 lower Helderberg group ; Becraft's mountain, Hudson", its differential 

 being found in the strong, sharp, subequal striae. The ornament of 

 the species, Brach. varistriatum, specially in its smaller forms, is 

 that of the well known Rafinesquina alternata of the Trenton 

 limestone and many coeval and later shells; that is, it consists of 

 coarser, filiform striae alternating with a group of subequal, finer ones. 

 This character is well expressed on specimens of Brach. m a j u s 

 through early stages and till the maximum of convexity is attained 

 and the pronounced deflection to the anterior margin begins. Thence 

 forward this character becomes modified. In the other species, Brach. 

 schuchertanum, however, early stages do not exhibit this arrange- 

 ment of the ornament, but start with the surface characters of the 

 var. aratum Hall, retain it for a brief period only, the resultant orna- 

 ment being distinct from that in any of the allied shells. No stronger 

 argument on behalf of the specific distinction of the var. arata could 

 be adduced. 



With regard to the peculiar structure of the hinge which charac- 

 terizes the genus Brachyprion, both of these Oriskany shells have 

 it well developed; the delthyrium is open and the denticulations of 

 the hinge-line are small and extend only about half way to the 

 cardinal angles. 



Shell of quite large size at maturity. Hinge straight and having 

 the full width of the shell ; marginal outline elongate lunate. Surface 

 of the pedicle-valve gently convex but strongly sloping in the urabonal 

 region, the greatest elevation and convexity of the valve being attained 

 in front of the center and following a line parallel to the margin. 



