12 



summit, which is somewhat flattened behind the mouth. The sur- 

 face, as it appears on our two specimens, one of which is unaltered 

 limestone and the other silicified, is smooth. There are no radiat- 

 ing lines or ridges, granules or pustules and there are no pores 

 that can be discovered with an ordinary magnifier. 



The four plates, in the first range, form a subhexagonal cup 

 about one- third of the length of the body, which has a small 

 hemispherical depression at the bottom for the insertion of a col- 

 umn. The six plates in the second range are of unequal size, 

 four are hexagonal, one pentagonal and one heptagonal; all are 

 longer than wide, except the heptagonal plate, which is fully as 

 wide as long. Each plate is subpyramidal externally, that is, pro- 

 duced centrally in the form of a node, which gives to the central 

 part of the body its hexagonal outline. A cast, however, would 

 not, probably, preserve this hexagonal outline and might be per- 

 fectly round. 



There are eight plates in the third range of unequal size and 

 different in outline. Two hexagonal plates stand nearly upright 

 and abut upon the mouth, with their superior angles between the 

 mouth and an arm opening on either side. Six of the plates rest 

 between the superior lateral sides of the plates of the second range 

 and two of them, each, truncate a single plate in the second 

 range. The latter two are the smaller plates in the range. All of 

 the plates of this range except where truncated by the mouth and 

 arm openings abut upon the plates of the vault. 



There is a subcentral plate on the vault, but the sutures between 

 the plates in our specimens are not distinct, and we are not cer- 

 tain, therefore, as to the number of plates surrounding it; appar- 

 ently there are only six, but probably there are seven. There are 

 only six armholes that are clearly distinguishable on the limestone 

 specimen, but there appear to be nine in the silicified specimen. 



This species is widely separated from all hitherto described, but 

 there is no doubt about the generic relations. 



Found in the Niagara Group, in Wayne county, Tennessee, and 

 now in the collection of Wm. F. E. Gurley. 



