214 GENERAL GEOLOGY. 



the sub-generic name of Archimedes, and which gave the 

 former name to this formation. Although the remains of 

 those two classes of mollnsks are common, and in some parts 

 of the formation even abundant, its crinoidal and fish remains 

 constitute its most important palseontological features. 



Radiates. Of corals, the genera Zaphrentis, Amplexus, 

 and Aulopora are found, two species of the former genus 

 being quite common, but crinoids are far more abundant than 

 all other radiates. Indeed the Keokuk limestone is decidedly 

 crinoidal, but not so conspicuously so as both divisions of 

 the Burlington limestone are. 



In an article published by the writer in 1860, an interesting 

 relation was shown to exist between these limestones of the 

 Sub-carboniferous group, by means of their crinoidal forms. 

 Thus, by comparing collections of these fossils from both 

 divisions of the Burlington limestone, together with those 

 from the Keokuk limestone, we find that they exhibit three 

 successive grades of general type. Taken as a whole, those 

 of lower Burlington limestone are of smaller size than those 

 of the upper, and these latter are generally smaller than those 

 of the Keokuk limestone. Again, those first named, have 

 generally a delicacy of construction and surface ornamen- 

 tation, together with a pleasing symmetry of outline, which 

 cause them to present quite a contrast with those of the upper 

 division, the aspect of the latter being more gross, and their 

 ornamentation coarser. In the Keokuk limestone, most of 

 the species are large, and they reach there a culmination -of 

 rudeness in their ornamentation and a strange extravagance 

 of form, quite characteristic of its crinoidal forms. 



With this culmination, crinoids cease to have much promi- 

 nence among the fossils of the Carboniferous system, although 

 they are absent from none of its formations. 



The genera of crinoids and other echinoderms are not so 

 numerous in the Keokuk limestone as they are in either of 

 the divisions of the Burlington limestone. It is proposed to 

 give a complete catalogue of these interesting fossils from all 

 the Iowa formations in a future report. 



