402 PALEONTOLOGY OF NEW YORK. 



an obscure fold which may sometimes be obsolete. Cincture scarcely per- 

 ceptible, sometimes producing a sliglit constriction in the inferior margin. 



Surface marked by fine concentric striae, which may be more or less fas- 

 ciculate, producing gentle undulations of the surface. No radiating striae 

 have been observed. 



Ligamental area narrow. Anterior muscular impression strongly marked. 



Five specimens measure respectively 29, 31, 46, 48 and 49 mm. in length, 

 and 12, 13, 17, 17 and 18 mm. in height. 



The shell is proportionally longer and narrower than any of the preceding 

 species, the form is more arcuate and there is no conspicuous fold on the post- 

 cardinal slope. 



Formation and localities. In the Chemung group, at Philipsburgh, and in loose 

 masses of sandstone near Portville and Olean, N. Y. 



Sphenotus rigidus. 

 plate lxvi, fig. 11. 



Cypricardiaf rigida. White and Whitfield. Proc. Bos. Soc. Nat. Hist., vol. viii, p. 300. 1862. 

 In part Sanguhwlites rigidiis (White and Whitfield), Hall. Prelim. Notice Lamellibranchiata, 2, p. 44. 



1870. 

 " " " " " Pal. N. Y., vol. V, pt. 1. Plates and Expla- 



nations : PI. G6, figs. 1-19. 1883. 



The figure is from the original specimen described.* It differs from S. con- 

 tractus in its more robust, shorter and more rigid form, more obtuse post-inferior 

 extremity, and more nearly vertical lower truncation of the posterior end. The 

 supposed clavicle corresponds with the posterior limit of the anterior muscular 

 impression as shown in fig. 9 of plate Ixvi. 



Formation and locality. In the Yellow sandstones, at Burlington, la. 



* The specimens here described as S. contraetus and 8. arcitattis wei-e identified by Mr. R. P. Whitfibld 

 as belonging to SaTiffuinolitex rigidux, White and Whitfield, and tluis refeiTed in the Preliminary Notice 

 of the Lamellibranchiata, 2 (above cited), and at the same time were thus arranged on the Plates for the 

 Pala;ontology of New York, then being lithographed. A cai-eful comparison of specimens with a cast of the 

 original of A. rigidus has shown the necessity of separating the Chemung 8})ecies, and recognizing in one of 

 the forms, the original of Cypricardia contracta, published in 1843. 



