CEPHALOPODA. 229 



grit are chiefly made up from the results of many years' collections in the 

 Helderherg mountains, and at Schoharie, for the State Museum. The few- 

 specimens in which surface characters are preserved will at once be apparent, 

 while the otherwise imperfect condition of the greater number will sufficiently 

 explain the difficulty of presenting a satisfactory monograph of the species. 



In the limestone coming above the Schoharie grit Ave have few species, and 

 comparatively few individuals, of the genus; and these, so far as known to 

 the writer, are in an imperfect condition, though often presenting the surface- 

 markings. All the collections made from the limestones within the limits of 

 the State of New York number less than fifty individuals. This fact may 

 seem remarkable, when it is known that several species of Gyroceras attain 

 their maximum development in the same horizon. 



At the base of the Hamilton group, the Marcellus shale affords six species 

 of the genus, associated with Nautilus and Goniatites. In the mass of the 

 .-aiiie group, in a higher position, we have at least twenty species, some of 

 which are extremely numerous in individuals. At the present time, however, 

 we must regard this group of shales and shaly sandstones as not sufficiently 

 explored to enable us to present a full account of its Cephalopodous fauna. 



In the Portage group the species and individuals yet known are not numerous, 

 thougli it is quite probable that, in favorable localities, additional species will 

 be found. The Chemung group has afforded few species, and these are frag- 

 mentary, and in an unsatisfactory condition of preservation. 



In comparing the Orthoceratites of the New York strata with those of 

 Bohemia, so amply and beautifully illustrated by M. Barrande, we find that 

 the surface-markings in the latter are almost universally imbricating lamellose 

 strne. Corresponding American forms have generally a reticulate surface. 



The curved forms, of which there are many in Bohemia, are very imperfectly 

 represented among the American species. The " breviconcs" of Barrande, are 

 scarcely known among the Orthoceratites of America, unless they may have 

 been confounded with Gomimiockras. 



The annulated forms in Bohemia are usually marked with imbricating 

 lamella? or lamellose striae, while in America many of the Lower Silurian 



