132 [Senate 



each side of the small mesial fold ; beak scarcely elevated above 

 the cardinal border, and, together with its linear area, incurved : 

 ventral valve the more convex, most prominent at the umbo, 

 from which it slopes regularly to the anterior lateral borders, 

 marked by about nine angular plications on each side of the 

 sinus, the lateral plications not reaching the beak, but coalescing 

 with a marginal ridge along the border of the area ; mesial sinus 

 angular, and rather sharply defined quite to the apex of the 

 beak ; area flat, rather high, transversely striated, obliquely 

 inclined from the hinge ; foramen very narrow ; beak scarcely 

 elevated above the margins of the area, not incurved. Surface 

 ornamented by fine undulating concentric lines of growth. 

 This shell may be distinguished from the last, as well as from other 

 somewhat allied forms, by its larger and more angular as well as less nu- 

 merous plications, and in having a distinct linear ridge along the margin 

 of (he area of the ventral valve. Its foramen is likewise narrower, and the 

 sinus deeper and more angular than in S. segmentus. 



Geological positio?i and locality. Limestone of the Upper Helderherg : 

 Stafford, Genesee county. 



Spirifer macrothyris. 



Pal. N.Y. Vol. iv. 



Shell large, transversely semioval, about twice as broad ais long ; 

 hinge nearly or quite equalling the greatest width of the shell, 

 angular or somewhat rounded at the extremities : dorsal valve 

 convex, rising in the middle into a narrow rounded mesial fold, 

 with eleven or twelve simple broadly rounded and slightly ele- 

 vated plications on each side ; beak extending but little beyond 

 the hinge, together with its narrow area distinctly incurved : 

 ventral valve the more depressed, plicated like the other, and 

 having a distinct somewhat broad mesial sinus, which becomes 

 much broader and deeper in front, imparting a sinuous outline 

 to the shell ; beak comparatively small, incurved ; area narrow, 

 somew^hat abruptly arcuate above ; sides parallel, extending to 

 the extremities of the hinge ; foramen large, broad triangular. 

 Surface marked by fine regular concentric lines of growth. 



