272 GE1TOKAL GEOLOGY. 



They are always found extremely compressed, indicating that 

 they were very thin. It has been suggested that the Petrodi 

 were dermal appendages of fishes related to the Rays. But 

 if this is the case, it is exceedingly difficult to account for the 

 absence of the slightest trace of the dental apparatus of 

 those fishes, which we might reasonably expect to find depos- 

 ited in the same strata. Prof. McCoy, however, has shown 

 that the microscopic structure of these teeth is essentially the 

 same as in all the Hybodont sharks, and which differs widely 

 from the simple structure of the dermal points that cover the 

 bodies of the Eays and sharks. 



The foregoing and other peculiarities commonly observed 

 in connection with the various strata of which the Middle 

 coal formation is composed, will be noticed in detail and 

 made more apparent in the descriptions following the 

 accompanying section, which presents a tabular view of 

 the formation in Iowa, as it is at present understood. 



3. GENERAL SECTION OF THE MIDDLE COAL-MEASURES. 



No. 1. The lowermost beds of the Middle coal-measures, 

 as exposed in the vicinity of the city of Des Moines, present 

 variegated red and blue shales, slightly arenaceous in places, 

 with one or two thin bands of impure limestone, more or 

 less irregular and nodular, with Clionetes mesoloba, Sjpiri- 

 ferina, Kentuckensis . At Des Moines, this stratum has an 

 exposed thickness of fifteen feet ; it is, doubtless, consid- 

 erable thicker. 



No. 2. Limestone, compact, gray and dark buff, usually 

 accompanied above and below by thin bedded layers, two to 

 three feet. This rock is seen at the quarries on Mosquito creek, 

 Dallas county, and on the Little Whitebreast, near Chariton, 

 Lucas county, at both of which localities it is well stored 

 with fossils, the following species having been obtained: 

 Aviculopecten rectilateraria, very abundant; Myalina, Swal- 

 lovi, Scliizodus, (sp. undet.), Productus muricatus, obscure fish 

 remains. The bed in Lucas county, referred to the above 



