316 S. WELLER— FAUNA OF THE FERN GLEN FORMATION 



1895. Ptyclwspira sexplicata H. & C, Paleontology of New York, volume 8, 



part 2, plate 83, figure 28. 

 1900. Retzia ? raricosta Rowley, American Geologist, volume 25, page 266, 



plate 5, figures 34-37. 

 1904. Ptyclwspira sexplicata Greger, American Geologist, volume 33, page 15. 



Description. — Shell subcircular in outline, usually a little wider than 

 long, but sometimes longer than wide, the valves subequally convex; 

 hinge line about one- third as long as the width of the shell, the cardinal 

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

 beak rather blunt, slightly incurved, pierced by a small, circular fora- 

 men; cardinal area small, slightly arched, the delthyrium occupying 

 nearly half its breadth along the hinge line ; delthyrium closed by a pair 

 of deltidial plates which are frequently destroyed in the specimens; 

 surface of the valve marked by from six to twelve strong, rounded plica- 

 tions which are separated by deep rounded grooves about equal in width 

 to the plications themselves; the two median plications are the strongest, 

 the lateral ones becoming successively smaller toward the cardinal ex- 

 tremities, the outermost ones sometimes being almost obsolete. Brachial 

 yalve more uniformly convex than the pedicle, its most prominent point 

 being near the center; the surface marked by from seven to thirteen 

 strong plications, corresponding with those of the opposite valve. Be- 

 sides the strong plications, the surface of both valves is marked by more 

 or less indistinct concentric lines of growth which sometimes become 

 crowded and conspicuous toward the margin of fully grown shells. 



The dimensions of two very perfect individuals from the Chouteau 

 limestone of Pettis county, Missouri, are: Length, 9 millimeters and 10 

 millimeters; width, 10 millimeters and 11 millimeters; thickness, 6.5 

 millimeters and 5.5 millimeters. 



Remarks. — This little shell is one of the less common members of the 

 Fern Glen fauna and usually occurs in a more or less crushed and im- 

 perfect condition. The above description has been made from Chouteau 

 limestone specimens from Pettis county, Missouri, where the species 

 occurs in a very perfect condition. The variation of the species consists 

 chiefly in the number of plications upon the shell, those with the larger 

 number being proportionally broader than the others, and in the con- 

 vexity of the valves. Retzia plicata, described by Miller as having from 

 ten to twelve plications, is not specifically different from White and 

 Whitfield's shell with "only about six plications/' In the Chouteau 

 limestone at Sedalia, Missouri, where Miller's type specimens were col- 

 lected, examples occur showing the whole range of variation of the spe- 

 cies as regards the number of plications. The most usual number of 



