346 INTRODUCTORY REMARKS. 



are Petrodus occidentalis, and spines like that figured on PI. II, fig. 3 and 3 a, 

 under the name of Listracanthus hisfrix. 



The occurrence of the so-called dermal plates of Petrodus, with the peculiar 

 spine above referred to in the same stratum, would seem to indicate that they 

 might have once belonged to the same species of fish, and this supposition seems 

 to be strengthened by the fact, that while these two forms are tolerably abun- 

 dant at some localities, all other traces of vertebrata are exceedingly rare in the 

 roof shales of this coal. 



The large spines, Physonemus gigas and Ctenacanthus Mayi, represented on 

 PI. II, figs. 1 and 2, were obtained from the upper division of the Burlington 

 limestone, the former from the quarries near Thayer's distillery, about one mile 

 below the City of Quincy, and the latter from Burlington, Iowa. More re- 

 cently we have obtained another specimen of the last named species from the 

 same limestone on Cedar creek, in Warren county, Illinois. The specimen of 

 Physonemus, the only one at present known, so far as we are aware, in America, 

 was found in the debris of the quarry, and the stratum in which it was origi- 

 nally embedded, could not be positively identified, but the Ctenacanthus Mayi 

 was obtained from the limestone layer known as the "fish bed" in the upper 

 division of the Burlington limestone. 



The remainder of the ichthyic material, still in hand, and upon the investi- 

 gation of which we are now engaged, indicates that this department of palaeon- 

 tology is by no means exhausted in this State, and we hope to present in a sub- 

 sequent volume of this report, some ten or twelve additional plates of these 

 very interesting fossils, illustrating at least fifty or sixty additional species, 

 which will enable us to extend our catalogue to something over two hundred 

 species of fossil fishes from the Carboniferous system alone, showing that our 

 western localities of Coal Measure and Lower Carboniferous limestone strata, 

 are far more productive in this interesting group of fossils than any other por- 

 tion of the earth's surface hitherto explored. A. H. W. 



