«I0 PALEONTOLOGY OF NEW- YORK. 



Trciuatospira costata ( n. g.)- 



Plate XXVIII- A. Fro. 4 a - «. 



Shell transversely elliptical, length lees than one-half the width : hinge 

 line slightly declining on each side of the centre ; cardinal extremities 

 rounded : valves very moderately convex in the middle, and flattened 

 towards the margins. Ventral valve with a well defined sinus below the 

 middle : beak moderately elevated, incurved, and tnmcated by a small 

 perforation.' Dorsal valve convex in the middle, with a broad scarcely 

 defined mesial fold ; cardinal margin thin, and closely pressed to the 

 opposite valve : beak incurved beneath the opposite, and filling a broad 

 pit, leaving on each side an undefined false area on the ventral valve. 



Surface marked by simple radiating costae, about fourteen or fifteen ^^or 

 more) on each valve, three of which are elevated on the mesial fold 

 and two in the depression ; the entire surface granulose or punctate, 

 the granules arranged, in concentric lines with a few imbricating 

 lamellae of growth. 



This species is well marked by its simple costse, which cover the mesial fold as 

 well as the lateral portions of the shell. It is extremely extended laterally, and 

 much appressed at the extremities and on the cardinal margins. Without careful 

 examination, this species would readily be mistaken for a Spirifer with the area 

 obliterated by pressure. 



Fig. 4 a, b. Dorsal and ventral valves. 



Fig. 4 e. Profile view. 



Fig. 4 d. Front view. 



Fig. 4 e. Enlargement of the surface, showing the granulose punctate character, which Lb 

 obscnre from the silicification of the shell. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains. 



