SOa PALiEONTOLOGY OF NEW-YORK. 



8|)irifer niacropleunis. 



Platk XXVII. Fio. la-p; and Plate XXVIII. Fio. 8 a - d. 



DtUhyrit macropleara : Conkad, An. Report on Palseontology of New-York, 1840, p. 217. 



— — Vanuxem, Rep. on Geology Third District New- York, 1843, pa. 120, f. 1. 



— — Mather, Rep. on Geology First District New- York, 1843, pa. 343, f. 1. 



Shell large, varying from semielliptical to semicircular or transversely 

 elliptical, ventricose : valves nearly equally convex ; hinge line often 

 scarcely equalling the greatest width of the shell. Area narrow. Fo- 

 ramen large. Ventral valve with a broad deep curved sinus and three 

 strong rounded plications on each side : beak moderately elevated 

 above the opposite, and abruptly incurved. Dorsal valve with a broad 

 rounded mesial fold and two strong rounded plications, with sometimes 

 a third one on each side. 



Surface marked by fine closely arranged radiating striae, which are 

 crossed by finer concentric ones ( the latter rarely visible)! 

 This species, in its general characters, is a representative of the type of a group 



of which S. radiaius and S. niagarensis are the earliest known forms; and is the only 



one of this character that I have seen in the Lower Helderberg group in New- York. 



PLATE XXVII. . 



Fig. 1 a. A young shell in which the plications are but faintly developed. 



Fig. 1 h, c. Dorsal and profile views of a specimen of medium size. 



Fig. \ d k h. Front and cardinal views of the same. 



Pig. 1 e,/, g. Dorsal, front and cardinal views of a very symmetrical form of medium site. 



Fig. 1 k. Ventral view of a larger individual. 



Fig. 1 /. Cast which is somewhat crushed from the base. 



Fig. 1 m, n. Interior of the ventral valve, and cast of the same. 



Fig. 1 o. Part of the exterior of a slightly weathered specimen, showing the extremity of 

 the spire. 



Fig. 1 p. Enlargement of the radiating striae as seen under a lens upon the surface of ordi- 

 nary specimens. 



PLATE XXVIII. 



Fig. 8 a, b, c. Casts of the ventral valve showing some variety in the forms of the muscular 

 impressions. 



Fig. 8 d. Enlargement of the surface striae, taken from a mould of the exterior in the shaly 

 limestone, where the shell has decomposed. 



[ This character is rarely well preserved on the surface of thefossil.] 



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

 group : Helderberg mountains ; Schoharie, Carlisle, Catskill, Hudson ; Penn- 

 sylvania, Maryland and Tennessee. 



