CLIOTHYRIDINA 481 



Brachial valve equally or a little more convex than the pedicle, greatest 

 convexity posterior to the middle, the surface curving abruptly from 

 the point of greatest convexity to the cardinal margin, and more gently 

 to the anterior margin, laterally from the median line the surface is at 

 first convex and then becomes straight or rather strongly concave, the 

 lateral slopes of the valve being compressed, in extreme cases strongly 

 so; the mesial fold broadly rounded, ill-defined laterally, most elevated 

 in front, in those specimens with the lateral slopes of the valve strongly 

 compressed the broad, ill-defined fold extends well towards the beak ; beak 

 strongly incurved, filling the delthyrium of the opposite valve. 



Surface of both valves marked by crowded, concentric rows of closely 

 imbricating, fine spines, which are usually more or less completely eroded 

 except towards the anterior and lateral margins of the shell. These 

 spines are in such close contact with the surface of the shell, at times, 

 as to give it the appearance of being finely costate towards the margin. 



Remarks. This species is most closely allied to C. hirsuta, and perhaps 

 might not be considered as distinct from it by some observers. It differs 

 from that species in being proportionally much thinner, in having the 

 broad, anterior, mesial flattening of the pedicle valve produced towards 

 the opposite valve in front, in the compressed lateral slopes of the 

 brachial valve, and in the finer surface spines. The species has been ob- 

 served only in the St. Louis limestone, where it sometimes occurs in 

 great numbers upon the surface of the limestone in partings between 

 beds. The specimens are apt to be much crushed and flattened, and the 

 true form of the shell is only occasionally found. 



Horizon. St. Louis limestone. 



CLIOTHYRIDINA PARVIROSTRIS (Meek and Worthen) 

 Plate LXXVIII, Fig. 25; Plate LXXX, Figs. 61-66 



1860. Athyris parvirostra Meek and Worthen, Proc. Acad. Nat. Sci. Phil., 



p. 451. 

 1866. Athyris planosulcata ? Meek and Worthen, Geol. Surv. 111., vol. 2, 



p. 254, pi. 18, figs. 8a-d. 



Description. Shell below medium size, lenticular, subcircular to sub- 

 pentagonal in outline, the length and breadth subequal, the greatest width 

 near the mid-length of the shell. The dimensions of a nearly complete 

 specimen are: length of pedicle valve 19 mm., length of brachial valve 

 18 mm., width 18.5 mm., thickness 11.5 mm. 



Pedicle valve most convex near or posterior to the middle, the surface 

 curving abruptly to the cardinal margin and more gently to the lateral 

 and anterior margins; beak small, incurved, perforated by a small sub- 

 circular foramen which encroaches upon the umbonal portion of the 



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