DESCRIPTION OP SPECIES BRACHIOPODA 317 



plications seems to be eight on the pedicle valve and nine on the brachial 

 valve, those adjacent to the cardinal line on each valve being rather faint. 

 The convexity of the valves varies considerably, the broader individuals 

 often being ranch thinner than the narrow ones, as is shown in the dimen- 

 sions of the two individuals given above. 



i >• : > ! i ; I ■ "•■■- 



RETZIA CIRCULARIS S. A. M. t 



Plate 12, figure 23 



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

 Indiana, page 316, plate 9, figures 32-34. 



Description. — Shell small, subovate in outline, the valves subequally 

 convex, the length and breadth subequal, the greatest breadth at or near 

 the middle, the postero-lateral margins meeting at the beak in nearly a 

 right angle. Pedicle valve with a small, pointed beak, the greatest con- 

 vexity posterior to the middle, the surface rounding to the margin in all 

 directions, but most abruptly toward the cardinal line; along its median 

 line the valve is slightly flattened or sometimes very slightly depressed 

 to form an obscure mesial sinus, the apparent sinus being mostly due to 

 the partial suppression of the median plication; the brachial valve with 

 its greatest convexity posterior to the middle, from which point the sur- 

 face slopes to the margin with a convex curve in all directions, most 

 abruptly posteriorly and postero-laterally ; surface of each valve marked 

 by from twelve to fifteen simple, subangular plications which are about 

 equal in width with the intervening furrows; no concentric markings 

 of the shell are visible. 



The dimensions of a nearly complete individual are : Length, 5.2 milli- 

 meters; breadth, 5 millimeters; thickness, 2.5 millimeters. 



Remarks. — This species is one of the less common members of the 

 Fern Glen fauna. It seems to agree in general form with the specimens 

 of Retzia circularis from the Chouteau limestone of central Missouri, 

 from where that species was originally described, but it differs from au- 

 thentic examples in the partial suppression of the median plication of 

 the pedicle valve, and in the somewhat finer plications of the Fern Glen 

 examples. The internal structures of the species are unknown, but it is 

 not improbable that it should be referred to the genus Ptycliospira along 

 with P. sexplicata. The shell also resembles Rliynclionella tuta S. A. M., 

 from Lake Valley, New Mexico, but judging from the figures and de- 

 scription alone, it seems to differ from that species in a manner similar 

 to its differences from the authentic examples of Retzia circularis. 



