612 



GEOLOGICAL SURVEY OF IOWA. 



anal side, wider than long. Third radials somewhat irregular- 

 ly hexagonal, supporting on the upper oblique edges two 

 subpentagonal supraradials : from the outer side of each of 

 these supraradials proceed two brachial plates, and from the 

 inner side a second and third supraradial plate, the latter 

 supporting two brachial plates; giving (as far as known) 

 three arms on each division, or six arms to each ray. 



First interradial plate large, hexagonal, with strong ra- 

 diating ridges, supporting two second interradials, and these 

 a series of four or five others ; two of which, on each side, 

 sustain the bases of the arms, lying against the last radial 

 and the supraradial plate. First anal plate a little smaller 

 than the first radial plates, supporting three plates in the 

 second series and four in the third series. Dome moderately 

 elevated, composed of a large number of tuberculiform plates, 

 as many as twenty of which occupy each of the superaxillary 

 areas : proboscis small, central. 



FIG. 93. 



The above diagram shows the form and proportions of the plates of the body and 

 the bases of the arms, as far as can be traced in the specimen Plate xv, fig. 5. 



