«0'J PALEONTOLOGY OF NEVV-YORK 



in Schoharie and at Ilainilton, Cazenovia and Matlisoii ( Madison county) ; but is 

 unknown in this formation in the western part of the State. There is a cast of this 

 species in the cabinet of Marietta Collejje, Ohio, which, I infer from it.s cliaiacter, 

 has been derived from the southwestern extension of the Hamilton gioup, perhaps 

 iu Virginia. 



Spiiifera niacrotliyrls. 



PLATE XXX. 



Spiri/rr maerothyris : Hali, Tenth Report on State Cabinet, p. 132. 1857. 

 " " Description of New Pal. Fossils, p. 92. 1857. 



Shell large, moderately gibbous, transverse, semielliptical, the width a 

 little more or less than twice the length : hinge-line equal to the 

 greatest width of the shell ; cardinal extremities rounded or angular, 

 and produced. Surface plicated. 

 Ventral valve less gibbous than the dorsal, regularly arcuate from front 

 to beak, and curving gently to the lateral and cardinal margins : sinus 

 well defined above, and reaching to the small and neatly incurved 

 beak, becoming wider and deeper below and much extended in front. 

 Area concave, of moderate width, and extended to the cardinal extre- 

 mities ; margin angular : fissure very large. 

 Dorsal valve the more convex, the greatest convexity about the middle 

 of its length ; mesial fold prominent, angular in its upper part and 

 rounded towards the front, giving a highly arcuate outline to the mar- 

 gin ; umbo much elevated and rising nearly as high as that of the 

 opposite valve ; beak incurved over the nearly vertical area, which is 

 about a line in width. 



The surface is marked by about ten or twelve strong low rounded pli- 

 cations on each side, which are simple from their origin, becoming less 

 prominent towards the margin of the shell : the first five or six on each 

 side of the fold and sinus occupy the greater pai*t of the anterior margin. 

 In well preserved specimens the surface is marked l)y fine closely 

 arranged concentric striaB, which are finely papillose from radiating 

 striae. The cast of the ventral valve presents a large ovate bilobed mu.s- 

 cular impression. 



