392 PALAEONTOLOGY OF ILLINOIS. 



twelve to fourteen more or less convex, or tumid pieces, 

 the first of which is hexagonal, about as large as the second 

 radials, and rests as usual on the upper truncated side of a 

 subradial; above these there are usually three arching 

 ranges of three each, and three or four other pieces still 

 farther up, more or less irregularly arranged. 



Vault flat, not quite equaling the greater breadth of the 

 body below, and composed of irregular tumid pieces, gen- 

 erally of rather small but unequal sizes, with a more or 

 less marked depression opposite each false arm ; opening, 

 apparently nearly central. False arms composed at their 

 bases of four rows of small pieces above, and two below, 

 arranged so as to enclose two distinct canals which do not 

 pierce any of the pieces themselves. 



Surface A r ery finely granular. Impressions at the corners 

 of the plates also sometimes cause the appearance of a ten- 

 dency to form a short obtuse radiating rib at each side of 

 some of the plates, but this character is too faintly marked 

 to attract attention. 



Hight of body, about 1.70 inch; breadth, 1.60 inch. 



The most marked feature of this species is its rather obovate form, 

 produced by the narrowness of its lower part and its convex sides, and 

 slight contraction near the top. The narrowness of its lower part re- 

 sults from the small size of its basal, subradial, and first radial pieces. 

 The deeply sunken character of its base also contributes to the same 

 result, as it is not near so wide as it would be if its pieces extended out 

 horizontally from the column, instead of being folded down with their 

 backs against it. It seems to resemble G. tiiberculosus, Hall (sp.), more 

 nearly in general appearance than any other species known to us, but 

 will be readily distinguished, not only by its different form, but by the 

 proportionally smaller size of its basal, subradial, and first radial pieces, 

 but more particularly by the structure of its false arms, which are 

 each composed at the base of six ranges of pieces, instead of only 

 two. 



Locality and position Upper division, Burlington group, Burlington, 

 Iowa. Lower Carboniferous. No. 379 of Mr. WACHSMUTH'S collec- 

 tion. 



