CLINTON GROUP. 
73 
All the specimens here figured occur in the upper green shale, and nearly all at Rochester. 
Figs. 5, 6, 8 and 8 a, occur in the lower green shale in Wayne county, being common to 
the two shales. 
]. Strophomena elegantula. — This is a beautiful semicircular shell, marked with strong 
elevated lines, having from four to six fine striae between each. It occurs in Wayne county, 
and at Rochester. This shell bears considerable resemblance to <S. transversalis. 
2. Strophomena corrugata. {Jour. Acad. Nat. Science, Vol. viii, p. 256, PL 14, fig. 
8.)—This shell is beautifully radiated, the rays dividing, a larger and smaller one alter¬ 
nating. On the hinge margin it is marked with a few oblique folds, which are frequently 
obsolete in well preserved specimens. This appears to be the fossil of which large numbers 
are imbedded, and completely silicified in the Pentamerus limestone. It occurs both in the 
limestone and shale. The specimen figured is from the upper green shale at Rochester. 
3. Strophomena cornuta. — Shell semicircular, finely and equally striated ; hinge line 
straight; each valve furnished with six small, stiff, diverging spines. This shell closely resem¬ 
bles the Leptcena lata of Von Buch, as figured in the Silurian System (PI. 5, fig. 13); but 
it is much smaller, and apparently more finely striated. The L. lata is found in the Ludlow 
rocks of England, which are equivalent to our Hamilton group. The S. cornuta occurs in 
a situation associated with fossils typical of the Caradoc sandstone of the Silurian System. 
This shell is abundant in the upper green shale at Sodus bay. 
4. Atrypa hemispherica {?) {Silurian System, PI. 20, fig. 7). — This little shell has from 
twelve to fourteen radii. It is usually much flattened, though one valve is much more convex 
than the other. It corresponds very closely with the figure and description given by Mr. 
Murchison. It occurs in such numbers in the upper green shale at Rochester, as to form 
two thin layers before referred to, where the shells retain their natural pearly lustre. This 
fossil occurs at Wolcott, Sodus, Rochester and Medina, as well as several other places. 
5 and 6. Littorina cancellata. — Fig. 5 is a young individual of this species, finely and 
beautifully cancellated over the whole surface. This marking is prominent in young speci¬ 
mens, but becomes obliterated in older ones, as is seen in fig. 6, where two views are given of 
an older individual of the same species. Some faint longitudinal strise are here visible, and in 
other specimens I have detected the crossbarred markings partially worn off. The shell is 
frequently quite smooth. It is abundant in the green shale at Sodus, and at Rochester ; also 
in the Pentamerus rock at Rochester and Medina. 
7. Posidonia ? alata. — Shell compressed, alated behind and rounded before, very inequi¬ 
lateral, wider than long; surface rather strongly marked with concentric lines of growth. 
Found in the upper green shale at Rochester. It is not an abundant fossil. 
8. Atrypa affinis, var. 1 — This shell presents the form and markings of some of the indi¬ 
viduals referred to that protean species. The portion within the shaded line is strongly ribbed 
and marked by lines of growth ; without this line, the margin is compressed, and the lines of 
growth scarcely defined. It often occurs, presenting this form, and appears like the A. affinis 
surrounded by a fringe. 
Geol. 4th Dist. 
10 
