COMPOSITA 489 



icle valve. It resembles C. Icevis, but besides being a somewhat 

 thicker shell it does not possess the distinct sinus of the brachial valve 

 and the consequent anterior emargination of that species, and the concen- 

 tric lines of growth are much more conspicuous. In its surface markings 

 the species most closely resembles C. trinuclea, but it may be easily dis- 

 tinguished from that species by the obsolete fold and slight sinus. 

 Locality. Patrick's Quarry, Middle Pabius River, Lewis Co.. Mo. 



COMPOSITA SUBQUADRATA (Hall) 

 Plate LXXXI, Figs. 1-15 



1858. Athyris sub quadrat a Hall, Geol. Iowa, vol. 1, pt. 2, p. 703, pi. 27, 



figs. 2a-d;p. 708, fig. 118. 

 1894. Athyris subquadrata Keyes, Mo. Geol. Surv., vol. 5, p. 92. 



Description. Shell of medium size, the width equal to, greater than or 

 less than the length, subovate or subquadrate in outline, the greatest 

 width usually a little anterior to the mid-length of the shell, the fold and 

 sinus well developed anteriorly. The dimensions of four specimens are: 

 length 27 mm., 25 mm., 25 mm., and 18.5 mm. ; width 27.5 mm., 26.5 mm., 

 24.5 mm., and 19 mm. ; thickness 17 mm., 17 mm., 17.5 mm., and 11.5 mm. 



Pedicle valve most convex posterior to the middle, the median line 

 arched from the beak to the front margin with a decreasing convexity 

 anteriorly, the surface curving abruptly from the umbonal region to the 

 cardinal margin and much more gently to the antero-lateral margins ; the 

 mesial sinus usually obsolete in the anterior half of the shell, originating 

 near the middle of the valve as a broad, shallow, ill-defined depression, 

 remaining broad and shallow to the front of the shell where it is more or 

 less produced in a lingual extension, the lateral slopes on each side of the 

 sinus convex ; the beak incurved, truncated obliquely to the plane of the 

 valve and perforated by a large, subcircular or subovate foramen ; car- 

 dinal area obsolete, the delthyrium broadly triangular, nearly filled by the 

 beak of the opposite valve. 



Brachial valve more convex than the pedicle, the greatest convexity 

 usually in front of the middle, the surface curving rather abruptly to- 

 wards the cardinal margin and much more gently towards the front; 

 mesial fold nearly or quite obsolete posteriorly, broadly- rounded and 

 more or less elevated in front, lateral slopes of the valve convex except 

 where they curve up into the sides of the median fold ; the beak strongly 

 incurved, filling the delthyrium of the opposite valve. 



Surface of both valves marked by concentric lines of growth which 

 become more numerous and sometimes crowded towards the front of the 

 shell. 



Remarks. This species has not infrequently been confused with C. trin- 

 uclea, as here interpreted, but it differs from that form in the larger size 



