74 
PALAEONTOLOGY OF NEW YORK. 
Observations. There is a noticeable variation in the internal characters of 
the species referred to this genus, but it does not appear to be of an essential 
nature, or of a greater degree than might be expected in different species; 
indeed this variation is strikingly apparent in different individuals of the same 
species. 
Of the interiors of the three known species of Leptobolus, that of L. occidens 
is not yet satisfactorily known, but those of both the other species are of not 
uncommon occurrence. L. lepis is found in the Utica slate of New York, at 
Holland Patent, and elsewhere, having been washed by thousands into the rill- 
marks or depressions in the sediment, *but upon cleaving the rock the shells are 
exposed with their interiors usually attached to the matrix, making it necessary 
to remove the scale-like shell in order to ascertain the internal characters. In 
the gray, muddy shale of the lower part of the Hudson group of Ohio 
(Utica horizon), the same species has accumulated in great numbers, and 
by breaking the rock the interiors are usually exposed; the specimens are, 
therefore, in a much more favorable condition for study. Leptobolus insignis, 
readily distinguished from L. lepis by the radiating striae on its internal surface, 
is found in much the same condition of preservation, but in the more compact 
layers of the Cincinnati or Hudson group. It will be understood that shells 
washed about as these have been, may often have lost the clear definition of 
their delicate interior impressions, but examples are not infrequent which 
retain with great sharpness all the internal details. 
The interior of the pedicle-valve shows a notably large cardinal area, which 
is sharply grooved. Beneath this area, in the bottom of the valve, is a broad 
depression extending nearly across the shell, and divided by a low median 
ridge, which bifurcates at its extremity, leaving between its branches a small 
central muscular impression. This latter feature is more clearly developed in 
L. lepis than in L. insignis, but in the latter species the entire depression is 
much more clearly defined, its ante-lateral margins being produced slightly for¬ 
ward. This impression is bounded on its sides by a crescentic muscular fulcrum, 
which extends, parallel with the margins, to the anterior portion of the shell. At 
a point back of their centers, each gives out a transverse branch extending in- 
