HUDSON-RIVER GROUP. 



303 



GASTEROPODA OF THE HUDSON-RIVER GROUP. 



Plates LXXXIII. & LXXXIV. 



Among the Gasteropoda of the Utica slate and the succeotling shales and sandstones, 

 there are few forms with which we are not already familiar in the Trenton limestone. 

 In the lower black slate we rarely find specimens of this order, a single species being all 

 that I have observed. In the succeeding strata we find a single well marked species of 

 Bellerophon, differing from those of the Trenton limestone ; and that peculiar form, the 

 Cyrtolites ornatus, which is unknown in any other position. A very small proportion of 

 the species known in the limestone reappear in this position, and in nearly all instances 

 the specimens are but poorly preserved. I have introduced a single well marked and 

 characteristic species from the western extension of the group, though it has not been 

 distinctly recognized in New-York. 



Almost all the specimens occur in the form of casts of the interior, ti»e shell being 

 rarely preserved; in consequence of which, there is some difficulty in identifying them. 



236. 13. MURCHISONIA GRACILIS. 



Pl. LXXXIII. Figs. 1 a, 6. , 



Reference Murchisonia gracilis, pag. ISl, pi. 39, figs. 4 a, b, c, of this volume. 



I am unable to find any marks of distinction between the slender forms so common in 

 the Trenton limestone, and those in the shales of the Hudson-river group, which will 

 enable me to refer them to distinct species. There are some slight variations observable even 

 in specimens from the same rock ; but these appear to be due to accidental causes, or the 

 different chararter of the enclosing material. This species is abundaiU in tlie shales and 

 calcareous portions of the sandstones of tliis group, but I have never been able lo obtain 

 one where the shell is preserved. 



Fig. 1 a. A specimen (a cast) from the calcareous sandstone. 

 Fig. 1 ft. A similar cast from the soft shales, at Loraine. 



Position and locality. This species occurs throughout the group, and is one of the most 



coiumon forms at Turin, Loraine, Washingtoin illc, Pulaski, and Rome. It is also found 



in the altered slates of the group, on the Hudson river, near Waterford. 



(State Collection.) 



