620 GEOLOGICAL SURVEY OF IOWA. 



plates, which, in the curvature, bring the summits of these 

 plates nearly as high as the upper side of the arm-bases. 

 First anal plate as long as the first radial plates, succeeded 

 by three longer plates. The area above broad and flattened, 

 with the aperture opening laterally. Summit marked by two 

 tuberculiform plates in the axils of the arms, and two others 

 in a line between these and the summit, with smaller plates 

 intervening. 



COLUMN consisting of alternating thicker and thinner 

 joints, which are striato-granulate on the exterior margin. 



This species has a close resemblance in general form to the A. tuberosus, but differs 

 essentially in the details of its structure, and somewhat in form of the body. The 

 second radial plates are hexagonal, while in the other they are quadrangular : the 

 third radial is proportionally a larger and stronger plate, while the first interradials 

 are much shorter than in A. tuberosus, and support two narrow second interradials 

 which reach to the second radial plate. 



A comparison of the diagrams giving the forms and proportions of the plates of 

 the two species will show these differences to be important. The anal side is not pro- 

 tuberant as in the other species, but presents a nearly flat area, with the aperture 

 opening laterally and farther from the summit. The dome is composed of a greater 

 number of plates, but with fewer large tuberculiform ones. 



A comparison with specimens in the collection of the late Dr. TROOST shows that 

 the characters already illustrated are those of the genuine JL. tuberosus. The original 

 of f. 1, pi. xvi, was received as a specimen of A. tuberosus ; but some differences being 

 observed, a comparison was instituted with other specimens from Keokuk, and these 

 differences found to be constant in several, while there is no evidence of a gradation 

 of the one form into the other. I must therefore regard them as distinct species. 



FIG. 97. 



The accompanying diagram represents the form and 

 proportions of the basal and first radial plates, with the 

 plates of one ray entire, including the brachial and arm- 

 plates to about the tenth range. The adjacent interradial 

 plates on the two sides show a marked difference when 

 compared with those of A. tuberosus. 



