ORTHIDES OF THE HAMILTON GROUP. 51 
or interrupted,' from the peculiar manner in which the pores open upon 
the surface. 
Interior of the ventral valve marked by a subcircular or broadly ovate 
flabellate muscular impression, which occupies more than half the 
length and breadth of the valve, and, in old specimens, is extremely 
thickened from its anterior margin nearly to the border of the palleal 
impression; the ovarian spaces distinctly pitted, pustulose or rugose, 
in curving ridges. Dental lamellae strong and prominent, merging below 
into the raised border of the muscular impression. 
Interior of the dorsal valve showing a strong prominent cardinal process, 
which is continued in a median ridge sometimes nearly to the front of 
the shell. In some specimens, low transverse ridges separate the ad¬ 
ductor muscular scars; while in other specimens, and particularly the 
older ones, this division is scarcely distinct, and the muscular impres¬ 
sion is broadly oval, with its lower margins showing foliate impressions 
as in the muscular impression of the ventral valve. 
This species reaches a larger size than 0. vanuxemi; often measuring more than 
one inch and three quarters in transverse diameter, while the largest specimens 
of 0. vanuxemi seldom measure more than one inch. It differs also in the character 
and strength of the radiating strise ; the muscular imprint in the ventral valve is 
usually broader and more strongly marked ; the cardinal and brachial processes 
of the dorsal valve are stronger, and directed towards the opposite valve; while 
these, in 0 . vanuxemi , are inclined forwards, or into the cavity of the shell. 
In specimens of the larger and medium size, the ventral valve is quite flat or 
concave below the middle. The tubular character of the strice is more strongly 
developed than in 0 . vanuxemi or 0 . leucosia; while in the finer punctate texture 
of the shell, there is no distinction perceptible. In all the authenticated specimens 
of this species, the cardinal line of the dorsal valve is straighter, and the beaks 
not approximating. The strong similarity between the three species above named 
will render it difficult in many cases to determine them. 
Geological formation and locality. In the calcareous shales of the Hamilton 
group, at Eighteen-mile creek and Hamburgh, Erie county; Alexander and 
Pavilion, Genesee county; York and Moscow, Livingston county; and on the 
shores of Canandaigua and Seneca lakes. 
