48 
PALAEONTOLOGY OP NEW-YORK. 
The interior of the ventral valve is marked by a large flabelliform 
muscular impression, which reaches from one-half to two-thirds the length 
of the shell. The central or adductor impression is sometimes simple, 
and sometimes longitudinally divided by a slight median ridge which is 
stronger below. In the older shells, the ovarian spaces are pustulose. The 
dental lamellae are strong and divergent, supported below by the ridge 
which margins the muscular impression. Vascular impressions are rarely 
seen, extending beyond the muscular area. Under a lens, the interior 
surface is distinctly punctate. In all well preserved specimens, the exte¬ 
rior shows minute tubular openings in the striae ; and when the striae are 
much worn, these also are seen to be tubular; while a farther wearing of 
the surface shows more distinctly the minutely punctate character of the 
shell. 
This species resembles the figures of 0. michilini of Laveille; but, compared 
with European specimens thus labelled, it presents many points of difference. 
The 0. vanuxemi is the most common and abundant species in the Hamilton 
group. The largest specimens have a length of about one inch and one-eighth, 
with a width of an inch and a quarter. 
Geological formation and locality. In the calcareous and arenaceous shales of 
the Hamilton group, in Central and Western New-York. Among the best locali¬ 
ties, are the shores of Cayuga, Seneca and Canandaigua lakes ; Moscow; Darien, 
and Eighteen-mile creek ; ranging through nearly the entire thickness of the 
group. In the more arenaceous shales farther east, it is common in the form of 
casts. It has been recognized in Maryland and Virginia; and specimens from 
Iowa have not been proved distinct from this species. 
Ortliis leucosia. 
PLATE VI. 
Orthis leucosia. : Hall, Thirteenth Keport on the State Cabinet,, 18G0, p. 80. 
Shell broadly ovate, greatest width below the middle, somewhat obtusely 
pointed at the beak; cardinal area short and small, being less than 
half the width of the shell : beaks approximate. Dorsal valve much 
the more gibbous, the greatest convexity above the middle, longitudi¬ 
nally marked by a median depression which is sometimes obsolete ; 
