HUDSON-RIVER GROUP. 303 
GASTEROPODA OF THE HUDSON-RIVER GROUP. 
Pirates LXXXIII. & LXXXIV. 
Among the Gasreropopa of the Utica slate and the succeeding 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 
Betieropuon, 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, the shell being 
rarely preserved ; in consequence of which, there is some difficulty in identifying them. 
236. 13. MURCHISONIA GRACILIS. 
Pu. LXXXIII. Figs. 1 a, d. 
Reference Murchisonia gracilis, pag. 181, pl. 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 character of the enclosing material. This species is abundant in the shales and 
calcareous portions of the sandstones of this group, but I have never been able to obtain 
one where the shell is preserved. 
Fig. 1 a A specimen (a cast) from the calcareous sandstone. 
Fig. 1 6. A similar cast from the soft shales, at Loraine. 
Position and locality. This species occurs throughout the group, and is one of the most 
common forms at Turin, Loraine, Washingtonville, Pulaski, and Rome. It is also found 
in the altered slates of the group, on the Hudson river, near Waterford. 
(State Collection.) 
