58 University Geological Survey of Kansas. 



find, upon examination of the Kansas Coal Measures forms, 

 that the length of the hinge and the height of the area, as well 

 as the relative convexity and bilobation of the brachial valve, 

 are quite variable. The diverging lines which pass from the 

 apex of the deltidium obliquely to the hinge line, nearly bisect- 

 ing the area, are common to all the well-preserved specimens 

 from the Coal Measures that I have examined. The same is 

 true of the vertical striation of the area between these lines. 



The area between the diverging lines is composed of two 

 large dental plates not entirely united to the remainder of the 

 hinge area. This is clearly shown by specimens with the area 

 crushed. In such cases it almost always breaks along these 

 lines with smooth fracture. 



They also present some striking peculiarities in the surface 

 markings of the shell. Upon a cursory examination there 

 seem to be two distinct kinds of surface markings — those shells 

 in which the striae are small, somewhat rugose, separated by 

 wider channels ; and those with more rounded, crowded, rugose 

 ridges with narrow valleys. Upon examination of several 

 specimens of different species, the difference in these respects is 

 found to be due, either to weathering or to wearing previous to 

 fossilization. The stride are rounded, thicker at the top of the 

 ridge than at the base, furrows and ridges crossed by numer- 

 ous fine, concentric lamellae, which are raised into rugae on 

 the ridges and nearly imperceptible in the furrows. In the 

 slightly worn specimens a portion of the top and sides of the 

 ridges are worn away, making them appear keel-shaped, with 

 wider furrows. The wearing of the shell does not seem to di- 

 minish the roughness of the striae very perceptibly, while it 

 seems to bring out the concentric markings of the furrows more 

 distinctly. 



