Description of species — brachiopoda 315 



Cardinal extremities arid a conspicuous mesial fold and sinus. Pedicle 

 valve strongly convex, most prominent posterior to the middle, the sur- 

 face curving strongly from the umbo to the margins, but most abruptly 

 to the cardinal margins; mesial sinus large and deep, rounded in the 

 bottom, defined on each side by a rounded ridge, in old shells produced 

 in front into a lingual extension of greater or less length; beak rather 

 prominent, incurved, in -close contact with the umbo of the opposite 

 valve, pierced by a circular foramen. Brachial valve strongly convex, 

 with a rounded mesial fold which becomes very prominent anteriorly in 

 old shells; lateral slopes of the valve more or less strongly convex, de- 

 pendent upon the age of the individual. Surface of both valves marked 

 by closely arranged, thin, concentric, imbricating lamellae, which are pro- 

 duced very regularly into fine, flattened spines, the spines of successive 

 concentric rows being arranged in radiating series so that the entire sur- 

 face, even when the spines themselves are in large part destroyed, presents 

 the appearance of being regularly and finely marked in a reticulate 

 manner. 



The dimensions of two examples are: Length, 17.5 millimeters and 16 

 millimeters; width, 22.5 millimeters and 21.5 millimeters; thickness, 15 

 millimeters and 11 millimeters. 



Remarks. — This species was originally described from the Fern Glen 

 formation of Saint Louis county, Missouri, and is highly characteristic 

 of the formation wherever it occurs. The species is quite different from 

 any other American athyroid shell, but should be compared with the 

 European Athyris squamigera De Koninck, with which it is possibly 

 specifically identical. In the present paper the species is referred to the 

 genus Cleiothyris not because the internal characters of the brachidium 

 have been observed, but because of the character of its surface markings, 

 which most closely resemble those of C. royssii; the concentric fringes of 

 spines in the two species, however, are quite different, the spines of 

 C. royssii being narrower and not being arranged in regular radiating 

 series, as is so conspicuously the case in C. proutii. 



The shell from Lake Valley, New Mexico, which Miller has described 

 as Spirifera temeraria is clearly a very close ally of Cleiothyris prouti 

 and will probably prove to be specifically identical. 



PTYCHOSPIRA SEXPLICATA (W. d W.) 

 Plate 14, figure 11 



1862. Retzia sexplicata W. & W., Proceedings of the Boston Society of Natural 



History, volume 8, page 294. 

 1894. Retzia pllcata S. A. M., Eighteenth Report of the Geological Survey of 



Indiana, page 316, plate 9, figures 24-31. 



