62 The Ohio Journal of Science [Vol. XXI, No, 2, 



direction, and 2.5 mm. wide in a direction parallel to the circumference 

 of the theca. Their radial cross-section is subtriangular. Along their 

 inner faces the}^ are abruptly vertical. Their upper faces curve down- 

 ward with even convexity as far as mid-height on the outer margin of 

 the plates; these upper faces are crossed radially by low ribs, usually 

 five, sometimes four, on each plate. The lower faces also are moderately 

 convex. The narrow intervals between the plates are occupied by a 

 darker substance which, originally, may have been flexible, permitting 

 the submarginal ring to be flexible. 



The submarginal ring of plates is bordered exteriorly by a marginal 

 band of imbricating plates. Those in contact with the ring usually equal 

 or slightly exceed 1 mm. in width, sometimes attaining a width of 

 1.5 mm.; they appear to be short but wide, with broadly convex free 

 margins. The other plates are successively narrower, those at the 

 margin of the theca usually being about half, or slightly more than half, 

 of a millimeter in width; they are longer than wide. 



The basal part of the specimen, within the submarginal ring, appears 

 to be formed by numerous plates from less than one millimeter to slightly 

 over one and a half millimeters in diameter. These are irregularly 

 convex, so as to produce moderate depressions at numerous points. 

 Their margins are too poorly defined to determine whether there is any 

 definite system in their arrangement. 



Locality and Horizon. — From the Orchard Creek shale, 

 near Thebes, Illinois. 



Specimen figured and described by Miller and Gurley as one of their 

 types of Cyclocystoides illinoiscnsis (Fig. 28 on Plate 5, Bull. Illinois 

 State Mus. Nat. Hist., 1895; also Plate I, Fig. 18 of the present paper). 

 This specimen was described as having 13 submarginal plates, forming 

 not much, if any, more than one-half of a circle. From this the inference 

 is drawn that the complete specimen had 24 to 30 plates. As a matter of 

 fact fourteen plates and half of a fifteenth are present, there are suffi- 

 ciently distinct impressions to indicate the former presence of three addi- 

 tional plates, and their total number could not have exceeded 20, 

 although apparently there is room only for 18 or 19. The radial cross- 

 section of the plates is subtriangular, with the inner face abruptly 

 vertical as in Cyclocystoides ornatus. The radially directed ribs on the 

 upper faces of the latter are absent, but this upper surface is so badly 

 worn in the Miller and Gurley specimen, here described, that the failure 

 of these ribs to appear has no diagnostic value. There are four or five 

 series of small plates in the marginal band surrounding the submarginal 

 ring. In the opinion of the present writer, this specimen, the second one 

 of those figured and described by Miller and Gurley, should be referred 

 to Cyclocystoides ornatus Savage, and the name Cyclocystoides illinoisensis 

 should be restricted to forms resembling Figure 27 accompanying their 

 original description. 



Miller and Gurley describe the plates of the marginal band as elon- 

 gated nodes. 



