CLIOTHYRIDINA 477 



CLIOTHYEIDINA INCRASSATA (Hall) 

 Plate LXXIX, Fig. 12 



1858. Athyris incrassatus Hall, Geol. Iowa, vol. 1, pt. 2, p. 600, pi. 12, 



fig. 6. 

 1895. Athyris incrassata Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 46, 



fig. 21. (Not pi. 83, fig. 39.) 

 1899. Athyris incrassata (?) Girty, Mon. U. S. Geol. Survey, vol. 32, 



p. 562. 



Description. Shell above medium size, subovate in outline, the length 

 and width subequal, or longer than wide, the greatest width near the 

 mid-length; the hinge-line much shorter than the greatest width, the 

 cardinal extremities rounded; fold and sinus strongly developed an- 

 teriorly. The dimensions of a nearly complete pedicle valve are : length 

 :42.5 mm., width 40 mm., convexity 10 mm. The dimensions of a 

 brachial valve are : length 40 mm., width 45 mm., convexity 15 mm. 



Pedicle valve moderately convex, arched from the beak to the front 

 margin along the median line, greatest depth near the middle, the surface 

 curving rather abruptly from the umbonal region to the cardinal margin, 

 much more gently to the antero-lateral margins, often somewhat com- 

 pressed towards the cardinal extremities ; mesial sinus obscure or obsolete 

 posteriorly, usually originating near the mid-length of the valve, rounded 

 in the bottom and ill-defined laterally, becoming deep at the anterior 

 margin; beak rather small and pointed, incurved, pierced by a sub- 

 circular foramen; cardinal area obsolete, the delthyrium broadly tri- 

 angular. 



Brachial valve more convex than the pedicle, the greatest convexity 

 near the middle, the surface strongly convex on each side of the fold 

 and sinus, the curvature most abrupt to the cardinal margin ; mesial fold 

 obsolete or nearly obsolete posterior to the middle, becoming strongly 

 elevated anteriorly, rounded on top, not sharply defined laterally; beak 

 strongly incurved. 



In the usual condition of preservation, the surface of both valves is 

 marked by more or less crowded, concentric lines of growth. When not 

 exfoliated these growth lines are produced into lamellae which are divided 

 by grooves or slits into fine ribs or spines. 



Remarks. C. incrassata is a close, ally of C. obmaxima, being distinguished 

 from that species by its less transverse form, and by the more anterior 

 position of the point of origin of the fold and sinus. It occurs commonly 

 in the Burlington limestone, where the allied species is rarely found, 

 and has not been observed in the Pern Glen or the Keokuk where C. 

 obmaxima is most abundant. Not infrequently examples of this species 

 are dark colored, sometimes almost black, especially in the umbonal 



