528 GEOLOaT OF OHIO. 



» 

 with the latter, but they shade off so imperceptibly into the other form 



that it is a question whether they ought to be regarded as distinct. It 

 seems to me probable that the succulent fronds, buried quickly in the 

 homogeneous material which has produced the thicker layers, have re- 

 sulted in one forming the casts, and that the same fronds abraded, 

 partly decomposed, and retaining only their fibrous structure, have left 

 their casts in the surfaces of the layers, where the deposition of material 

 was arrested, so that the same plant has left apparently two distinct 

 forms of impressions. The thickness of the casts of the spirals in the 

 thick layers, and the amount of carbonaceous matter deposited in the 

 cavities shows that the fronds had considerable thickness. All the casts 

 here observed would also indicate that the plant consisted of a single 

 frond, making a spiral of about one and a half turns, and not the tapering 

 Archimedean screw figured by Prof Hall in Appendix D. of the Sixteenth 

 Annual Report of the State Cabinet of Natural History of New York. 

 It is possible, however, that these Ashland county forms are distinct from 

 those described by Prof. Hall, and are to be referred to a new species. 



