CLIOTHYRIDINA 475 



Surface of both valves marked by closely arranged, thin, concentric, 

 imbricating lamellae, which are produced very regularly into fine, flat- 

 tened spines, the spines of successive concentric rows being arranged in 

 radiating series so that the entire surface, even when the spines them- 

 selves are in large part destroyed, presents the appearance of being 

 regularly and finely marked in a reticulate manner. 



Remarks. This species is not closely allied to any other American 

 member of the genus. Its nearest allies are found in the European Athyris 

 squamigera DeKon. and Athyris roissyi Leveille. 



Horizon. Chouteau limestone and Fern Glen formation. 



CLIOTHYRIDINA OBMAXIMA (McChesney) 

 Plate LXXIX, Figs. 1-11 



1861. Athyris obmaxima McChesney, Desc. New Spec. Foss., p. 80. 

 1863. Spirigera pectinifera Swallow, Trans. St. Louis Acad. Sci., vol. 2, 



p. 88. 



1874. Spirigera obmaxima White, Prelim. Rep. Inv. Fo.-s., p. 17. 

 1877. Spirigera obmaxima- White, U. S. Geol. Surveys W. 100th Mer., vol. 



4, p. 92, pi. 5, fig. 12. 

 1895. Athyris incrassata Hall and Clarke, Pal. N. Y., vol. 8, pt, 2, pi. 83, 



fig. 39. (Not pi. 46, fig. 21.) 

 1909. Cleiothyris incrassata Weller, Bull. Geol. Soc. Am., vol. 20, p. 313, 



pi. 14, figs. 8-10. 



Description. Shell large, transversely subelliptical in outline, greatest 

 width near the mid-length of the shell, the hinge-line much shorter than 

 the greatest width, the cardinal extremities rounded, fold and sinus 

 strongly developed towards the anterior margin. The dimensions of a 

 nearly perfect individual are : length 40 mm., width 52 mm., thickness 

 24 mm. Some of the largest examples have a width of 75 mm. or more. 



Pedicle valve rather strongly convex, strongly arched from beak to 

 front along the median line, the greatest prominence posterior to the 

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

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

 sometimes a little compressed towards the cardinal extremities ; mesial 

 sinus obscure or obsolete at the beak, but usually originating in or near 

 the umbonal region, rounded in the bottom and ill-defined laterally, 

 growing rapidly stronger anteriorly, often becoming profound near the 

 anterior margin and produced in a lingual extension of greater or less 

 length; beak rather small and pointed, incurved, in contact with the 

 umbonal region of the opposite valve, pierced by a subcircular, oblique 

 foramen ; cardinal area obsolete, the delthyrium broadly triangular, con- 

 nected at its apex with the foramen, almost entirely filled with the beak 

 of the opposite valve. 



