4a« PALEONTOLOGY OF NEW-YORK. 



figured have a length of three inches, and I liave fragments of others wliich have 

 been much larger. 



The geological range of the genus, as at present known, is from the upper part of 

 the Lower Helderberg group, through the Oriskany sandstone, and into the Upper 

 Helderberg limestones. 



In its geographical range, it is known from Gaspe in Canada East, to Virginia and 

 Tennessee (and probably occurs in Alabama), and westerly from New- York through 

 Canada West, Mackinac island, Ohio, Illinois and Missouri. 



ItciisseKieria oroides. 



Plate CIV. Fio. 1-4; and Plate CV. Fia.l - 6. 



Meganteru ovoxda : Hall, Regents' Report for 1856, p. 102; Palssozoic Fossils, 1857, p. 62. 



Ttrebratula ovoidea : Eaton, Geological Textbook, 1832, p. 45. ( Not Sowerbt, 1812.) 



T. -ptrovcdia : Eaton, lb. id. ( Not Sowerbt, 1825.) 



Jhrtfpa elongata : Conrad, Annual Report on the Palajontology of New- York, 1839, p. 65. 



Not Pentamtrua dongatia of the Onondaga limestone, Vanu.tem, Geol. Report, 1842, p. 132, f. 1. 



Alrypa elongata : Vanuxem, Mathek and Hall, Geol. Reports, 1843. 



Shell ovoid, elongate-ovate or elliptical-ovate in outline, gibbous or 

 compressed, broadest above the middle, abruptly rounded towards the 

 cardinal end, narrowing to the front, which is often depressed and 

 rounded or subtruncate : no traces of a sinus in either valve. In gib- 

 bous specimens, the edges of the valves are incurved, and the sides 

 vertically flattened or a little concave, often slightly contracted near 

 the front. Ventral valve the larger, most gibbous in the umbonial region : 

 beak (in old specimens) somewhat obtuse, clearly incurved upon the 

 opposite. Dorsal valve convex, less elevated than the other, most pro- 

 minent along the middle or a little above it, forming a low semielliptical 

 arch from beak to front, sloping very gradually to the sides, which (in 

 old specimens) are often so abruptly inflected as to form a distinct 

 angular ridge extending from near the beak two-thirds of the way to 

 the front : beak obtuse, scarcely incurved. 



Surface marked by regular simple radiating strias, sometimes crossed 

 near the borders by distinct concentric wrinkles : structure punctate. 



