188 GENERAL GEOLOGY. 



Fossils. The most conspicuous and characteristic fossils 

 of this formation are brachiopod mollusks and corals. They 

 belong to the following named species : Spirifer pennatus, S. 

 Parryanus, Strophodonta demissa, Atrypa reticularis, A. 

 aspera, Ortliis Iowensis, and Acervularia Damdsoni. Other 

 species are abundant in some places, but those named have a 

 range throughout the formation in Iowa. The following 

 species found in this formation in Iowa, are also found in the 

 Hamilton shales in New York, namely: Stropliodonta 

 demissa, S. fragilis, Atrypa reticularis, A. aspera, Ortliis 

 Vanuxemi, Spirifer frimoriatus, and Cyrtia Hamiltonensis . 



IN" ear the town of Rockford, Floyd county, very great 

 numbers of well preserved specimens of the fossils of this 

 formation are found in its marly clays. Several species are 

 abundant there besides those here enumerated, but they seem 

 to have a limited geographical range. A species of SmitMa 

 replaces Acervularia here, for which the name S. Woodmani 

 is proposed, in honor of H. T. Woodman, Esq., of Dubuque.* 



The coral Acervularia Damdsoni occurs near Iowa City in 

 CDnsiderable abundance solidified, with pure calcareous mate- 

 rial and consolidated in the rocky strata. These receive a fine 

 polish and make beautiful cabinet specimens, but are always 

 too small to be of any practical use as marble. It has been 

 known under the names of " Iowa City marble" and " Bird's- 

 eye marble." 



*This species differs somewhat conspicuously from S. Verrilli, of Meek, in the 

 uniformly larger size of the calyces, the great prominence of their margins above the 

 intermediate spaces, and the width of those spaces between the calyces. 



