NIAGARA GROUP. 
107 
miformis, Yon Buck, sur les Spirifers et Orthis, t. 1, f. 7. Silurian Researches, p. 630, pi. 
13, f. 10.) Obcordate, deeply two-lobed, striated longitudinally; lower valve convex, with 
an incurved beak, upper one flat or slightly concave ; area triangular, large in proportion to 
the size of the shell. 
Our specimen is smaller than the one figured by Murchison, but it agrees so closely with 
his figure and description, as well as with the figures of Von Buch and Hisinger that there 
remains no doubt of its identity. This shell resembles the D. varica , Conrad, Jour. Acad. 
Nat. Sci. Vol. viii. p. 262, pi. 14, f. 20; but in that shell the valves are nearly equally con¬ 
vex, and the hinge line less extended. 
Locality —Wolcott, Sweden, Monroe county. 
5. Orthis flabellulum {a)? {Silurian Researches, pi. 21, f. 8. O. callactis? Dalman).— 
Semi-oval, with 24 to 28 simple, rounded, smooth radii, which are crossed by a few lines of 
growth; the radii continue strongly marked quite to the beak, their breadth being equal to 
the spaces between them ; hinge line slightly arched, a little less than the width of the shell; 
upper valve flat, the lower one slightly convex. Length 8 lines, width one inch. 
In Murchison’s figure the ribs are broader than in our shell; the description corresponds 
with the exception that there are oftener 26 than 24 radii. I have seen a shell which is ap¬ 
parently identical with this one, from the limestone of the Clinton group, associated with 
Pentamerus oblongus. It differs from a similar shell in the Trenton limestone in the greater 
number of radii, and the less convexity of the lower valve. The radii in the latter are often 
bifurcate, while they are never so in the former. 
Locality —Lockport; Rochester. 
6. Orthis canalis {Silurian Researches, p. 630, pi. 13, f. 12, a.; also pi. 20, f. 8. Orthis 
elegantula? Dalman. Yon Bucii sur les Spirifers et Orthis, pi. ii, f. 3, 4 and 5).— Semi-oval, 
finely radiated, radii dividing towards the margin ; lower valve very convex, with a produced 
incurved beak; upper valve nearly flat or with a slight depression along the centre, which is 
rarely characteristic in our specimens; hinge line shorter than the width of the shell. 
This shell is every where found in the shale of the Niagara group, and is one of its most 
characteristic fossils. It is somewhat variable in form, being often nearly circular ; and is 
evidently identical with the figure of Murchison, and is probably the same shell figured by 
Dalman. Von Buck remarks in his description of this shell, that, it is found of precisely the 
same form as the Swedish specimens, but a little smaller; at Castle Hill, Dudley, and at 
Wenlock Edge, Shropshire. 
In the Delthyris shaly limestone of the New-York system, there is a shell almost precisely 
similar in form to this one, but more robust; the lower valve is very convex, almost carinated 
in the centre, the upper one slightly convex, and marked with a depression along the middle ; 
hinge line longer in proportion to the shell. The similarity of form in these two shells which 
are really distinct, renders it a matter of interest to ascertain the relative position of the Swe¬ 
dish, English and American species, and whether the former may not be identical to the larger 
one here mentioned. 
14 
