98 
PALAEONTOLOGY OF NEW-YOKK. 
This species is closely allied to the S. hemispherica, differing in the surface 
marking, and to some degree, in the muscular impressions; and it is impossible 
to determine how far these modifications may extend, without a large series of 
specimens in better preservation than usually found. In the Hamilton group, the 
species is subject to considerable variety of form and convexity, but is always 
readily recognized among the other species of the group. In the collections there 
ifc a single example of a dorsal valve, which is moderately concave for a little more 
than half its length, with some obscure concentric wrinkles; beyond this, it is 
deflected almost at right angles to the plane of the upper part of the shell. The 
muscular impressions are shorter and wider than usual, and the cardinal process 
is shorter and stronger. In comparison with the prevailing forms, this might have 
been regarded as a distinct species. 
Geological formation and locality. This species occurs rarely in the Cornife- 
rous limestone in New-York, and is very common in the soft shales of the Hamil¬ 
ton group in all the localities from Cayuga lake westward to Lake Erie. It is 
apparently limited to the upper beds of the group, or to the Moscow shale, and 
is comparatively rare below that horizon. At the present time, it is unknown to 
me in any position higher than the Hamilton group. 
Strophodonta perplana, 
PLATES XI, XII, XVII & XIX. 
Strophomena perplana : Conrad, Jour. Acad. Nat. Sciences, Vol. viii, pa. 257, pi. 14, f. 11. 1842. 
Strophomena pluristriata : Conrad, ut sup. p. 259. 
Strophomena delthyris : ut sup. pa. 258, pi. 14, f. 19. 
Strophomena crenistria : Hail, Report Fourth Geological Dist. New-Yorlc, p. 171, f. 4. 1848. 
Strophomena ( Strophodonta ) fragilis : Hall, Tenth Report on State Cabinet, p. 143. 1857. 
Strophodonta fragilis : Hall, Report Geological Survey of Iowa. pa. 496, pi. 3, f. 6a, 6c. 1858. 
Shell semielliptical; the length varying from two-thirds to three-fourths 
the width, which is from half an inch to two inches; slightly concavo- 
convex, and often nearly flat : hinge-line equalling or often a little 
greater than the width of the shell below; the extremities usually 
somewhat salient, except in extremely old shells. Margins of the shell 
often a little contracted just below the hinge-extremities, making the 
width less than below; but the sides are frequently nearly straight 
for half their length, and the front broadly rounded with the margin 
attenuate. 
