UTICA SLATE AND HUDSON-RIVER GROUP. 
301 
half way to the base; hinge line straight; posterior extremity obliquely truncated ; shell 
uniformly convex ; a round elevated ridge extending from the beak to the posterior basal 
margin, between which and the cardinal line a narrow portion of the shell is closely 
compressed. 
This species is very abundant in the green shales just above the black shale, and is also 
often found in the higher arenaceous parts of the group. The thin compressed portion on 
the posterior dorsal margin is often covered, so that the form is the same as fig. 9 a , which 
shows only the convex part of the shell. Sometimes the impressed line before the hinge is 
not distinctly visible, particularly in the folded and glazed slates of this group along the 
Hudson river; but in most instances it is a reliable character. In the decomposing arenaceous 
portions of the group, this is one of the most abundant fossils, associated with crinoidal 
joints, fragments of Trinucleus , and other fossils. 
Fig. 9 a. A specimen of the arenaceous slate, with several individuals of this species, associated with 
crinoidal columns, etc. 
Figs 9 b, c, d. Other individuals from different localities, showing a slight difference in form. 
Fig. 9 e. A specimen from the altered slates, preserving both valves. 
In its geographical distribution, this species is coextensive with the group, and is un¬ 
known below the Utica slate, or above the terminal sandstone of the Hudson-river group. 
Position and locality. This species occurs in all parts of the group, in the following 
localities : Turin and Martinsburgh, Lewis county ; Loraine, Jefferson county ; Pulaski 
and Washingtonville, Oswego county; near Rome, Oneida county; Waterford, Saratoga 
county, and other places. 
190. 2. NUCULA 1 POSTSTRIATA. 
Pl. LXXXII. Figs. 10 a, b. 
Reference pag. 151, pl. 34 of this volume. 
This species has already been given under the Trenton limestone, where it first occurs. 
In the Hudson-river group it acquires a greater development, and is both larger and more 
numerous. It rarely preserves the shell, however, and we are forced to depend on casts of 
the interior for our examinations. The strong diverging striae upon the posterior slope are 
usually marked upon the cast, which, with the subtriangular form of the shell, is sufficient 
to distinguish it from others of the group. The broader or more quadrangular form of the 
specimen in the Trenton limestone is due to the preservation of the shell, which gives the 
posterior slope a greater elevation. 
Fig. 10 a. A small specimen in the ferruginous sandstone. 
Fig. 10 b. A larger specimen, preserving very distinctly the striae on the posterior slope. 
Position and locality. This species occurs both in the shaly and arenaceous part of the 
group, at Loraine and Pulaski, though it is a comparatively rare form. {State Collection.) 
