CLIOTHYRIDINA 483 



1894. Cliothyrig Eoyssi Hall and Clarke, Int. to Study of Brach., pt. 2, 



pi. 35, fig. 10 (not fig. 9). 



1895. Cliothyris Royssi Hall and Clarke, Pal. N. Y., vol. 8, pt, 2, pi. 46, 



fig. 24 (not fig. 23). 



ID LI. Cleiothyris hirsuta Morse, Proc. Ohio State Acad. Sci., vol. 5, p. 388, 

 fig. 15. 



Description. Shell below medium size, usually longer than wide, the 

 greatest width near or posterior to the mid-length, sometimes wider than 

 long, suborbicular or subovate in outline, the hinge-line much shorter 

 than the greatest width, both valves convex, the brachial valve usually 

 more convex than the pedicle and sometimes notably so. The dimensions 

 of an average specimen are : length 19.5 mm., width 20.5 mm., thickness 

 12 mm. 



Pedicle valve moderately convex, the greatest convexity posterior to 

 the middle of the valve in the umbonal region, the surface curving ab- 

 ruptly from the umbonal region to the cardinal margin and gently 

 towards the lateral and anterior margins, often compressed or even con- 

 cave towards the lateral margins; beak not prominent, but slightly ex- 

 tended beyond that of the opposite valve, only slightly incurved, perfor- 

 ated by a subcircular, oblique foramen which opens into the delthyrium 

 posteriorly; mesial sinus obsolete in smaller individuals and sometimes 

 obscure in full grown examples, but often well developed towards the an- 

 terior margin of the valve where it is rather narrow and of moderate 

 depth, rounded in the bottom and ill-defined laterally. 



Brachial valve more convex than the pedicle, the greatest convexity 

 posterior to the middle, the surface curving rather abruptly from the 

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

 to the lateral and anterior margins, the convexity extending out to the 

 lateral margins or sometimes slightly compressed laterally; the beak in- 

 curved beneath that of the opposite valve, and more or less completely 

 filling the delthyrium ; mesial fold obscure except sometimes near the an- 

 terior margin, when best developed it consists of a narrow, mesial flat- 

 tened area more or less illdefined laterally, and not distinctly elevated 

 above the general surface of the valve except sometimes near the anterior 

 margin. 



The surface markings of both valves consist of broad, thin, concentric 

 lamellose expansions of the shell which are divided nearly to the base 

 into flattened spines, about three of which occupy the space of one milli- 

 meter. 



Remarks. This species is one of the conspicuous members of the lower 

 Chester faunas, and may be readily recognized on account of the greater 

 convexity of the brachial valve. In shale beds the surface ornamentation 

 is not infrequently more or less perfectly preserved, but in the limestones 

 these characters are all destroyed by exfoliation in removing the speci- 



