LOWER HELDERBERG ROCKS. 
CsEMJS EATONIA (n.g.). 
For description and illustration of tliis genus, see the same under Oriskany 
sandstone. 
Eatonia medialis. 
Plate XXXYII. Fig. 1 a - y. 
Atrypa medialis : Vanuxem, Geological Report of the Third District, 1843, pa. 121, f. 4. 
Shell transversely oval, suborbicular or subquadrate : hinge line nearly 
straight, and forming a very obtuse angle at the beaks. Dorsal valve 
much larger than the ventral, greatly elevated in the middle (especial¬ 
ly near the front), declining with a gentle curve towards the hinge and 
very abruptly towards the sides. Ventral valve flat or concave, depressed 
in front so as to form a broad and profound mesial sinus : beak very 
small, pointed but not prominent, incurved, perforate at the extremity. 
Surface marked by from twelve to sixteen broad rounded rarely bifurca¬ 
ting plications, four of which usually occupy the summit of the mesial 
fold of the ventral valve, and about three the bottom of the sinus in 
the dorsal valve : entire surface (in well-preserved specimens) marked 
by fine radiating striae; and rarely by a few imbricating lines of growth. 
The muscular impression in the ventral valve moderately large, ovate, 
very distinctly defined by a prominent border, and marked by longitudi¬ 
nal slightly radiating plications : near its centre is the small cordiform 
longitudinally striate impression of the adductor muscle. 
Associated with this species are a few forms, which, although differing materially 
from it, I am at present inclined to regard as merely extreme varieties of the same 
species. Some of these are given on the same plate ( See fig. la- g). In some in¬ 
stances (such as 1 c, d, f k g), the plications are almost entirely obsolete, and the 
valves are compressed together around the front and lateral margins. 
The fine longitudinal striae, as well as the finer concentric striae, are rarely pre¬ 
served upon the specimens which I have seen; though its occurrence upon a few 
perfect specimens prove this character to have originally existed. In the Eatonia 
peculiaris and E. singulars, the fine longitudinal striae are characteristic of the 
[ Paleontology III.] 31 
