120 [ Senate 



This species, in its general characters, resembles C. subhemispherica ; 

 but all the individuals I have seen are smaller, the radiating striae or 

 plications are less prominent and more broadly rounded, proportionally 

 stronger and fewer in number and more divergent, and remarkable in their 

 divergent bifurcation. In a single small specimen which preserves, almost 

 entire, two of the spines, the outer one is nearly as long as the whole length 

 of the hinge-line. 



In its diverging spines, this species resembles C. Jconz7icJciana, Pbatten 

 & Norwood ; but that species has double the number of striae, and half 

 as many cardinal spines. The specimens present some variation in different 

 localities, but all agree in the principal characteristics. 



I have specimens which are essentially undistinguishable from this 

 species, in a hard limestone from the " Devil's Bake-oven," Illinois. The 

 strise are, however, slightly less divergent, though corresponding in number. 

 Of two specimens among these, showing the spines imperfectly, both are 

 divergent iu the same manner as in the New- York specimens. In the western 

 locality it is associated with Chonetes carinata, C.pusilla and Strophomena 

 laticosta, as well as other fossils characteristic of the Hamilton group. 



Geological position and locality. In limestone a few miles southeast of 

 Buffalo, associated with Chonetes glalra, and in shales of the Hamilton 

 group on Canandaigua lake. In limestone, as cited above from Illinois, of 

 the age of the Hamilton group. 



Pentamerus ARATUS. 



Pal. N.Y. Vol. iv. 



Atrypa arata^ Conkad : 1841, Ann. Rep. Pal. N.T. p. 55. 

 Atrypa octocostata, Conrad : Idem. 



Shell arcuate-ovoid : dorsal valve mucli the smaller, depressed- 

 convex : ventral valve extremely elevated, forming almost a 

 semicircle from beak to front ; beak strongly incurved ; foramen 

 large, triangular. Surface marked by somewhat strong unequal 

 angular plications, which bifurcate very irregularly. 



A well-marked peculiarity of this species, in its mature condition, is 

 the extremely elevated arcuate ventral valve, giving it much the aspect of 

 a Productus. Of the other valve, little is known, further than that it is 

 much depressed. 



