CONTRIBUTIONS TO PALEONTOLOGY. 83 



SPIROPHYTON CRASSUM ( n. s.). 



PLATE II. FIG. 4. 



Frond spiral; disc abruptly depressed towards the centre, and less 

 concave towards the outer margin. Substance of the frond ridged, 

 radiating from the centre in fasciculi which expand and curve 

 towards the outer margin, where the surface becomes more even : 

 margin distinctly defined. 



This is a strong growing species, which attains a large size. In a 

 specimen about five inches in diameter, the margins of the volutions 

 are separated by a little more than half an inch of intercalated 

 stony matter, while the convexity of the volution is nearly an inch. 



The species occurs in greenish gray shaly sandstone below the Carboni- 

 ferous conglomerate at Cuyahoga falls, Ohio. 



I have a very similar species from the Chemung group of New-York, 

 which differs in having the ridges or fasciculi more sharply defined upon 

 the upper surface of the frond, which is nearly flat till within an inch of 

 the centre, where it is suddenly depressed. The specimen is imperfect, but 

 the single- disc before me has been, when entire, at least eight inches in 

 diameter. 



The preceding forms are illustrations of a natural and very pecu- 

 liar group among the numerous forms of marine vegetation, which 

 abound in some of the Upper Palaeozoic rocks of New- York and the 

 adjoining States. Their interest consists, not more in indicating a 

 peculiar group, than in the fact that, so far as at present known, 

 they characterize formations of a certain age, beginning with the 

 base of the Upper Helderberg group, and marking those strata which 

 we have regarded as of unequivocal devonian age. In this respect, 

 their occurrence may be found of advantage elsewhere, as indicating 

 strata of similar age. 



In other regions, however, where the line between Devonian and 

 Carboniferous is not so well defined as in New- York and to the 

 westward, these forms may be found to have a greater vertical range; 

 and I have not at present evidence for asserting that they do not 

 occur in the Lower Carboniferous shales of Pennsylvania. 



The object of this short notice will have been accomplished, if it 

 induce observation upon the mode of growth, nature, and geological 

 range of those peculiar forms. 



