194 PALJiONTOLOGY OF NEW-YORK. 



Spirifera grierl. 



PLATE XXVII & PLATE XXVIIL 



Spiriftr gritri ; Hail, in Tenth Keport on the Stale Cabinet, p. 127. 1857. 

 •• ■' ) New Species ol'Palaozoic Fossils, p 8". 1867. 



Shell gibbous, transversely oval or sirbquadrilateral, sometimes longitu 

 dinally ovate, the proportions of length and breadth being very varia- 

 ble ; hinge-line usually shorter than the width of the shell, with the 

 cardinal extremities rounded : valves subequally convex. 

 Ventral valve gibbous or ventricose, most convex above the middle and 

 nearly opposite the center of the hinge-line, and sloping very abruptly 

 to the lateral margins ; sometimes regularly arcuate in the entire 

 length, and often arched in the upper part and nearly straight below. 

 Umbo prominent and much elevated above the hinge-line : beak more 

 or less extremely incurved over the high arcuate area, which has a 

 length of from one-half to nearly two-thirds the width of the shell ; 

 mesial sinus wide and deep, subangular in the lower part. 

 Dorsal valve regularly arcuate, the greatest convexity near the middle 

 and regularly curving to the lateral margins : mesial fold prominent, 

 sometimes rounded, but usually more or less distinctly angular ; beak 

 small, slightly incurved over a nearly vertical narrow area. 

 Surface marked by six, eight or ten more or less rounded simple plica- 

 tions on each side of the mesial fold and sinus ; while there are three 

 or four distinctly bifurcating or dichotomous plications upon the fold 

 or sinus, giving six or seven at the margin of the shell. In perfect 

 specimens, the surface is covered by fine concentric lamellose striae, 

 which are crossed by delicate radiating striae. 



This species is distinguished from most of the allied forms by its simple strong 

 plications on each side of the mesial fold and sinus, while those occupying the 

 latter are smaller and bifurcating. Sometimes the middle plication on the summit 

 of the mesial fold is simple, in which case the fold is quite angular ; while in 

 other instances it bifurcates, leaving a longitudinal depressed line on the middle, 

 giving it a more rounded outline. It is only on specimens which have suffered no 

 injury by wearing or exfoliation, that the fimbriate appearance of the concentric 

 markings is viiiible.- 



