CHONETES 83 



sinus; the umbo a little prominent and projecting slightly beyond the 

 cardinal margin; the beak small and incurved; the cardinal margin 

 sharply defined and bearing about four spines on each side of the beak; 

 cardinal area not observed. 



Brachial valve only moderately concave, flattened towards the cardinal 

 extremities, the greatest concavity posterior to the middle. Internally 

 the cardinal process is small and is flanked by a pair of widely divergent 

 ridges which become nearly parallel with the cardinal margin and reach 

 about one-third the distance to the cardinal extremities, and in whose pos- 

 terior side the dental sockets are excavated ; in front of the socket ridges a 

 well-defined median septal ridge reaches to about the middle of the valve, 

 and also a pair of ridges, one on each side of the median ridge, which 

 originate at the base of the median ridge, with a slight divergence, and 

 curve towards the lateral margins, distally attaining a length of about 

 two-thirds that of the median ridge. 



Both valves marked by fine, rounded, radiating costse, separated by 

 narrower intercostal furrows, the costee increase by division on the ped- 

 icle valve and by intercalation on the brachial valve, from 3 to 4 occupy 

 the space if 1 millimeter, about 170 being present altogether upon the 

 specimen whose dimensions have been given. Crossing the costa? are 

 several irregularly distributed lines of growth of variable strength. 



Remarks. This species is most nearly allied to C. illinoisensis but it 

 attains a larger size and has a much coarser eostse, there being only 3 

 or 4 in the space of 1 millimeter, while in C. illinoisensis there are 6 in 

 the same space. 



Horizon. Pierson limestone of the Kinderhook. 



CHONETES CHESTEKENSIS n. sp. 

 Plate VIII, Figs. 31-34 



Description. Shell concavo-convex, the length about two-thirds the 

 width, the greatest width along the hinge-line, the cardinal extremities 

 angular; lateral margins nearly straight or slightly concave in front of 

 the cardinal extremities, convexly curved anteriorly and passing with 

 unbroken curvature into the anterior margin which becomes nearly 

 straight in its central portion. The dimensions of two specimens are: 

 length of pedicle valve 8.2 mm. and 8.7 mm., length of brachial valve 

 7.1 mm. and 7.8 mm., greatest width 12.6 mm. and 12.5 mm., convexity 

 2 mm. and 2.5 mm. 



Pedicle valve depressed-convex, the greatest convexity near the middle, 

 the surface curvature a little more abrupt to the anterior margin than 

 towards the beak, compressed towards the cardinal extremities; beak 

 small, scarcely produced beyond the cardinal margin; cardinal area low, 

 nearly flat except towards the apex where it is slightly concave, lying 



