122 Carl O. Dunbar, 



stride of which ten to twelve occupy a space of 2 mm. at the ante- 

 rior margin of the shell. The striae increase by both intercala- 

 tion and bifurcation, and there is considerable variation from 

 subequal striae in some specimens to distinctly fasciculate striae in 

 others. Pedicle valve strongly convex, with a slight undefined 

 median fold. Beak small and incurved over the cardinal area. 

 Dorsal valve also strongly convex and but little shallower than 

 the pedicle. A distinct but narrow median sinus extends entirely 

 to the beak. Interior characters closely resembling those of D. 

 subcarinata. 



Dimensions of a mature individual: Length, 7 mm.; width, 7.5 

 mm. ; depth, 4 mm. 



Discussion: This beautiful little species closely resembles the 

 young of D. subcarinata, with which it is associated. It is 

 slightly more finely striated and distinctly more gibbous, how- 

 ever, than the young of that species. It has a matured and 

 dwarfish appearance. The subequal convexity of its valves will 

 distinguish it from other described species of this small size. 



Occurrence: Birdsong shale member of the Linden. At the 

 Swayne's mill locality, at exposures on Big Lick and Birdsong 

 creeks, and at other localities in Benton and Decatur counties. 



Dalmanella rockhousensis, n. sp. 

 Plate II, figs. 6-8 

 Description: Shell small, subcircular in outline, width equal to 

 or very slightly greater than the length. Hinge-line shorter than 

 the greatest width of the shell, cardinal extremities rounded, 

 cardinal area shorter than half the width of the shell and strongly 

 curved. Ventral valve rather strongly convex, prominent in the 

 umbonal portion, but curving evenly to the front and sides with- 

 out any fold. Beak small and incurved over the cardinal area. 

 Dorsal valve gently convex, marked by a sinus which is very 

 narrow but distinct at the beak and widens toward the anterior 

 margin, where it is shallow and not sharply defined. Surface 

 covered with very fine radiating striae, which increase rapidly 

 by intercalation and frequently show a distinctly fasciculate 

 arrangement, coarser strize being separated by three or four finer 

 ones. About ten striae in a space of 2 mm. along the margin. 



