74 PALEONTOLOGY 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 Leptocolus, that of L. occidens 

 is not yet satisfactorily known, but those of both the other species aie 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 tlie 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 notabl}' 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 b}- 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- 



