310 PALEOZOIC PALEONTOLOGY. 
The dimensions of a nearly-perfect specimen are: length, 9 mm. ; 
width, 11.5 mm.; thickness, 7.5 mm. 
Remarks.—Several specimens of this little shell have been observed 
in the upper-portion of the New Scotland beds and in the Becraft lime- 
stone. They resemble the illustrations of R. formosa in the New York 
Paleontology, but are much smaller than the mature forms of that 
species, although they are evidently adult shells. 
EATONIA MEDIALIS (Van.). 
Plate XXXVL., Figs. 16-23. 
1859. Hatoma medialis Hall, Pal. N. Y., vol. ITD., p. 241) piaie 
figs. 1 a—y. 
Description.—Shell transversely subelliptical, subcircular or sub- 
quadrate in outline. Pedicle valve depressed-convex on the umbo, flat- 
tened or concave toward the lateral margins, depressed in front in a 
deep mesial sinus, which is more or less produced into a lingual exten- 
sion at nearly a right angle to the plane of the valve; the beak not 
prominent, incurved over that of the opposite valve. Internally the 
muscular impression of the pedicle valve is rather large, ovate, dis- 
tinetly defined by a prominent border and marked by longitudinal, 
slightly-radiating plications; near its centre is the small, cordiform, 
longitudinally striated impression of the adductor muscle. Brachial 
valve strongly convex, with a more or less elevated mesial fold toward 
the front; greatest elevation of the valve on the mesial fold at or near 
the front margin. Surface of each valve marked by from twelve to 
nineteen broad, low, rounded plications, which frequently continue to 
the beak, but which are sometimes obsolete beyond the middle of the 
valves. About three of the plications usually occupy the sinus of 
the pedicle valve, with four upon the fold of the brachial valve. In 
well-preserved specimens the surface is also marked by fine, radiating 
strie and sometimes by a few concentric lines of growth. 
The dimensions of a nearly-perfect specimen are: length, 20 mm. ; 
width, 22.5 mm.; thickness, 15 mm. Those of another specimen are: 
length, 18 mm. ; rade 22.5 mm.; thickness, 12 mm. 
