LAMELLIBRANCHIATA. 443 



Paracyclas tenuis. 



PLATE LXXII, FIGS. 20-2-2 ; and PLATE XCV, FIG. 26. 

 Paracyclas tenuis. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations ; PI. 72, fig's. 20-23. 1883. 



Shell small, sub-circular ; length and height about equal ; margins regularly 

 curving. Cardinal line short. 



Valves moderately convex. 



Beaks a little anterior to the middle, small, closely appressed, scarcely 

 rising above the hinge-line. Post-cardinal slope curved and rapidly declin- 

 ing backward, limited by the ligamental groove, which is very distinctly 

 marked. 



Test extremely thin. Surface marked by very fine concentric striae, which 

 are sometimes aggregated into fascicles toward the pallial margin. 



Four specimens' measure respectively 7, 8, 9 and 10 mm. in length, and 

 7.5, 7, 8, 5 and 9 mm. in height. 



This species is distinguished by its small size, and very finely striated sur- 

 face, without distinct undulations. 



Formation and localities. In the shales of the Hamilton group on the shores 

 of Skaneateles, Cayuga and Seneca lakes, N. Y. 



Paracyclas Chemungensis, n. sp. 



PLATE XCV, FIG. 23. 



Shell large, sub-orbicular, erect; length and height nearly equal; margins 

 regularly rounded. Cardinal line short. 



Valves depressed-convex below and convex above. 



Beaks sub-central, rather prominent. Umbonal depression very oblique 

 to the cardinal line. 



Surface marked by comparatively even, thread-like striae, which are some- 

 what fasciculate on the lower and posterior portions of the shell. 



One specimen has a length of 30 mm. and a height of 29 mm. 



