PALEONTOLOGY OF NEW- YORK. 



Spirifera ziczac. 



PLATE XXXV. 



DtlthyrU zigxag i Hall, Geol. Report Fourth District, pp. 200 and 201. 1843. 

 Spiri/tr clio : HALL, in Thirteenth Report on the State Cabinet, p. 93. 1800. 



Shell transverse, gibbous or ventricose, semielliptical in outline, with 

 the hinge-line equalling or greater than the width of the shell below ; 

 the cardinal extremities truncate or auriculate. Surface plicate. 



"Ventral valve the more convex, the greatest convexity being somewhat 

 above the middle, arcuate from beak to front ; sides regularly curving 

 to the lateral margins : sinus deep and wide towards the front, rounded 

 or flat in the bottom, produced in front, and sharply defined by the 

 adjacent angular plications, which are stronger than the others ; beak 

 incurved. Area elevated, concave, with the margin angular and curv- 

 ing to the cardinal extremities. 



Dorsal valve moderately convex at the sides ; the mesial fold abruptly 

 elevated, rapidly expanding below, flattened upon the summit, and 

 usually marked by a mesial depression. The lateral portions of the 

 shell curve towards the front, and are flattened or concave towards 

 the cardinal extremities. 



Surface plicated by about eight, ten, or even twelve strong angular 

 costae, those towards the cardinal extremities being less elevated and 

 often obscure. The entire shell is marked by fine concentric striaB, 

 which at intervals appear to be crowded together into stronger imbri- 

 cating lamellae, sharply arched and elevated upon the costaB and upon 

 the mesial fold, and usually having a slight retral curve in the bottom 

 of the sinus. In well preserved specimens, fine radiating striae are 

 visible. 

 The teeth are short and not very strong. The dental plates form a 



ridge along the margin of the fissure, and extend to the bottom of the 



cavity in its upper part, and margin the muscular area for only a short 



distance. • 



