82 Trans. Acad. Sci. of St. Louis. 



specimens, and it is rather singular that in each of these the 

 posterior ear has been destroyed so that its form cannot be 

 determined. The specimen illustrated is the best preserved 

 and largest one of the types, having a height and length of 

 13 mm. each. The smallest of the type specimens is but 

 8 mm. in height, while a fragmentary specimen not included 

 among the types is 17 mm. in height. The most characteris- 

 tic features of the species are the fine radiating striae with 

 the variable number of raised ribs, each of which is covered 

 with striae similar to those between. These ribs are variable 

 in number, from three to six or seven. In some specimens 

 they are much more conspicuous than in others, and those near 

 the center of the shell are always most conspicuous. Some- 

 times the ribs are nearly obsolete. 



AVICULOPECTEN CAROLI Will. 

 PI. III. f. 4. 



Shell subcircular, the height and length each 25 mm., 

 nearly equilateral, the greatest convexity 6 mm. at a point a 

 little above the middle. Hinge-line shorter than the shell. 

 Beak central, scarcely elevated above the hinge-line. An- 

 terior ear of the left valve slightly convex, sharply depressed 

 from the body of the shell ; its anterior margin nearly straight 

 above and forming approximately a right angle with the hinge- 

 line, rounded below where it forms a shallow angular notch 

 with the margin of the shell below. Posterior ear of the 

 same valve not so sharply separated from the body of the 

 shell ; its posterior extremity acutely angular, and the pos- 

 terior margin forming with the margin of the shell below a 

 shallow, rounded notch. The anterior, basal and posterior 

 margins together form something more than a nearly per- 

 fect semicircular curve. Surface of the shell marked by 

 about sevent}'-five raised costae towards the margin, which 

 vary greatly in size. These costae increase in number by 

 division and by intercalation, so that the number is much less 

 towards the beak. The larger costae are from three to five 

 times the width of the smaller ones. The larger ribs pro- 

 duced into spines on the margin of the shell. The surface 

 is also marked by indefinite concentric lines of growth. The 



