268 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



If asked why we do not find some of the prevailing forms of the Upper 

 Helderberg, we Icnow no better reply than the following : A friend and 

 accomplished geologist writes, " You ought to find, if your rock is Cor- 

 niferous, some characteristic moUusks as Euomphaliis De Cewi, Conocar- 

 clium trigonale, Pentcmierus aratus, Paracydas proavia, etc." In the same 

 letter he specifies as exclusive and diagnostic species of the Hamilton 

 of Ohio, Spirifera mucronata, Tropidoleptus carinatus, Pterinea jiahellum, 

 Nyassa arguta, etc. It is remarkable that in the Hamilton of Iowa, here 

 or elsewhere developed, not one of these fossils find place. We are neces- 

 sitated to rely on an entirely different series to determine the Hamilton of 

 Iowa. We look for Orthis lowensis, Spirifera pennata, Spirifera aspera, 

 Spirifera subattenuata, forms unknown to the Hamilton of Ohio. The 

 inference is valid, that if we are necessitated to look to Iowa for the 

 characteristic fossils of the Hamilton, why may we not look to Iowa for 

 the characteristic species of the Corniferous. 



As aids in the identification of our rock, we have first, characteristic 

 fossils of the Helderberg ; secondly, the occurrence of closely representa- 

 tive species ; and thirdly, the general aspect of the whole as a whole. 

 Under the first head we have the Gypidula loiviuscula, Hall, figured from 

 the horizon of the Upper Helderberg of Iowa. The Eenssellceria 

 Johanni, Hall, is also from the Upper Helderberg of Iowa. Our Gono- 

 cardium, is undistinguishable from the Conocardium trigonale, Hall, of 

 the Corniferous limestone of New York. As closely representative spe- 

 cies, our Gyroceras, seem to have their affinities with those figured 

 from the Corniferous in the Geological Report of Ohio. A Paracydas 

 here occurs, closely resembling the Paracydas proavia of the Cornife- 

 rous, the genus, as such, raainly confined to the Upper Helderberg. The 

 massive plating of our Oanoidea, with its array of stellate tubercles, at 

 least recalls the description of the Macropetalicthys Sullivanti, Newbury, 

 of the Ohio Geological Reports. The general aspect of the whole series 

 of fossils is widely different from that of the Hamilton. 



We do not disguise the fact that intermingled with the above fossils, 

 and especially through the lower parts of the rock, occur many of the 

 same fossils that are found in the Hamilton. Elsewhere they are com- 

 mon both to the Hamilton and Upper Helderberg, and are of no strati- 

 graphical importance. Such, for instance, are the Atrypja reticularis, 

 Atrypa rugosa, Athyrus vittata, Strophodonta demissa, and otliers. The 

 position taken by Prof. Hall, and maintained solely on lithological 

 grounds, that the Upper Helderberg is developed in our vicinity, seems 

 thus fully supported by paleontological evidence. With a knowledge of 

 the fossils above enumerated, there is every reason to suppose Prof. 

 Worthen would not hesitate to refer them to the same geological horizon. 



If there had been no natural lithological division, if these fossils were 

 only on the surface, if one or two species were alone represented, if they 

 were insignificant in characte'r, their evidence might possibly seem of 

 little weight. But the opposite is true in each particular. There is a 

 well recognized iiatural distinction. The fossils extend through a thick- 



