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PALAEONTOLOGY OF NEW-YORK. 
Spirifera ziczac. 
PLATE XXXV. 
Dclthyris zigzag : Hall, Geol. Report Fourth District, pp. 200 and 201. 1843. 
Spirifer clio : Hall, in Thirteenth Report on the State Cabinet, p. 93. 1860. 
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 striae, 
which at intervals appear to be crowded together into stronger imbri¬ 
cating lamellae, sharply arched and elevated upon the costae 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. 
