482 MISSISSIPPIAN BRACHIOPODA 



valve; delthyrium filled with the beak of the opposite valve, joined with 

 the foramen at its apex ; mesial sinus wanting, the mesial portion of 

 the valve sometimes slightly flattened, and the anterior margin a little 

 produced. 



Brachial valve about equally convex with the pedicle, the greatest con- 

 vexity near or a little posterior to the middle, the surface curving rather 

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

 anterior margins; mesial fold wanting, the mesial portion, of the valve 

 sometimes slightly flattened; the beak strongly incurved beneath that 

 of the opposite valve and nearly filling the delthyrium. 



The surface of both valves marked by thin, concentric, closely imbri- 

 cating, lamellar extensions which are divided into flattened spines, al- 

 though, as commonly preserved, these surface characters have been 

 removed by exfoliation. 



Remarks. This species is most closely related to C. hirsuta and C. 

 sublamellosa. From the first of these it differs in its larger size, usually 

 in its more subpentagonal outline, and often in the slight extension of the 

 anterior margin of the pedicle valve, although without the development of 

 a mesial sinus. In size the species is similar to C. sublamellosa, but it differs 

 from that species in having the valves subequally convex instead of 

 having the brachial valve notably more convex than the pedicle. The 

 species has sometimes been referred to the European Athyris planosulcata, 

 but that species differs from C. parvirostris and other members of the 

 genus Cliothyridina, in having the concentric lamella? of the shell entire, 

 not divided into spines, a condition which, so far as external characters 

 are concerned, would throw that species into the genus Atrypa in its strict 

 sense. The concentric lamellas of A. planosulcata, however, are distinctly 

 ribbed, and these ribs are only one step removed from the concentric rows 

 of spines in Cliothyridina. Exfoliated specimens of A. planosulcata and 

 C. parvirostris would perhaps be indistinguishable, since the general pro- 

 portions of the two shells are much alike. 



Horizon. Keokuk limestone. 



CLIOTHYRIDINA SUBLAMELLOSA (Hall) 

 Plate LXXX, Figs. 31-60 



1858. Athyris sublamellosa Hall, Geol. Iowa, vol. 1, pt. 2, p. 702, pi. 27, 



figs. la-c. 



1861. Athyris obvia McChesney, Desc. New Spec. Foss., p. 81. 

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



p. 89. 

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



p. 89. 

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



