314 S. WELLER FAUNA OF THE FERN GLEN FORMATION 



Description. — Shell biconvex, transversely subelliptical in outline, the 

 hinge line much shorter than the greatest width of the shell, the cardinal 

 extremities rounded. Pedicle valve most prominent on the umbo, the 

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

 lateral and antero-lateral margins; the median portion of the shell is 

 depressed in a rounded, ill denned sinus which originates on or just in 

 front of the umbo and becomes profound anteriorly ; the curvature of the 

 surface along the median line is strong, the anterior portion of the me- 

 dian sinus being directed at nearly a right angle to the plane of the 

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

 umbonal surface of the opposite valve. Brachial valve most prominent 

 along the median line in the anterior half of the shell, where the rounded 

 mesial fold is rather strongly elevated; lateral slopes convex, most abrupt 

 toward the cardinal and antero-lateral regions of the shell. Surface of 

 both valves, as usually preserved, marked by fine, more or less regular, 

 imbricating lines of growth which are often somewhat wavy along their 

 free margins. When perfectly preserved, these concentric lines were pro- 

 duced throughout as thin, flat spines lying in close apposition to the sur- 

 face of the valve. 



The dimensions of a somewhat imperfect individual are : Length, 40 

 millimeters; breadth, 56 millimeters; thickness, 26 millimeters. 



Remarks. — As this species sometimes occurs in the somewhat shaly 

 beds of the Fern Glen formation, the nature of its surface markings 

 may be clearly determined, although in no cases have the spinose exten- 

 sions of the concentric lamellae been observed except in small patches. 

 These markings are clearly those of the genus Cleiothyris. As the spe- 

 cies occurs in the superjacent Osage limestones, the surface of the shell is 

 uniformly more or less exfoliated, so that the fine surface details are 

 destroyed. 



In its general form and surface characters this shell closely resembles 

 Spirifer glabristria Phillips, which is placed among the synonyms of 

 Athyris royssii by Davidson, and it is altogether probable that the Amer- 

 ican and European specimens are members of a single species, although 

 it is not so certain that the glabristria should be included under Cleio- 

 thyris royssii. 



CLEIOTHYRIS PROUTI (Swallow) 

 Plate 14, figures 12-15 



1860. Spirigera proutii Swallow, Transactions of the Academy of Science of 



Saint Louis, volume 1, page 649. 

 1894. Athyris proutii Keyes, Missouri Geological Survey, volume 5, page 91. 



Description. — Shell transversely subelliptical in outline, with rounded 



