FOSSILS OF THE KEOKUK GROT P. 499 



GEMS AttARICOCRIXUS, Tro.M. 

 AcARicocRrsus WHITFIELDI, Hall. 



PI. 12, Pi;;. 1 a. b. PI. 15, Fig. 8. 

 Agaricoerinut Whitjieldi, HALL, 1668. Iowa Report, p. 621. Supp. same. PI. 3, Fig. 5, 1860. 



BODY attaining a rather large size, depressed subhemis- 

 pkerical, the vault being moderately convex, and the under 

 side so very deeply concave, that when the body is placed 

 upon a level surface, with the under side down, it rests upon 

 the first braehial pieces. Basal pieces very small and 

 entirely hidden by the column. First radials rather small, 

 wider than long, and hexagonal in form. Second radials, 

 except on the posterior side, slightly smaller than the first, 

 and all quadrangular excepting one on the anal side, which 

 has one of the upper angles (sometimes) truncated so as to 

 make a fifth angle. Third radial plates considerably larger 

 than the first or second, wider than long, three of them 

 hexagonal, one pentagonal, and one with a general hex- 

 agonal form, but one of the upper angles sometimes trun- 

 cated, so as to make a seventh angle. * First braehial 

 pieces very large, of different forms; second generally very 

 wide and short, and upon these, and in part upon the first, 

 the arms commence. First interradial plate more or h->-> 

 elongated, and irregularly octagonal or heptagonal in form, 

 and supporting in the next range two very narrow, much 

 elongated pieces extending up between the arms so as to 

 connect with the vault. First anal plate slightly longer 

 than the first subradials. and supporting on its short upper 

 truncated side an elongated hexagonal piece ; while its 

 superior lateral sloping sides each supports a somewhat 

 larger, elongated octagonal plate, which like the middle 

 piece extends up and connects with another range of five 

 smaller pieces, reaching the vault. 



*Prof. HALL describes these plates as being two hexagonal, and three pentagonal; this, however, 

 is certainly an error, as we know from the typical specimen now before us. His diagram, however, 

 .t* their form correctly. 



