144 PALEONTOLOGY OF KENTUCKY. 



the ventral ; it is very narrow, almost linear. The planes of the two cardinal 

 areas are inclined to each other in such a manner as to form an angle of more 

 than ninety degrees ; this angle of the areas is, however, different in one and 

 the same specimen according to the distance from the cardinal extremities ; 

 near the latter it is smallest and increases gradually, sometimes rapidly, to- 

 wards the beaks, below which, in some individuals, they form no angle at all, 

 but fall ih one and the same plane. 



The surface of both valves is ornamented by radiating and concentric striae. 

 Of the radii, some ten to twelve are much stronger and more elevated than the 

 balance, but they maintain their prominence only inside of the umbonal region; 

 leaving this, they bifurcate directly, which process is repeated about half-way 

 between umbo and front ; in addition to this increase by bifurcation, a further 

 enlargement in the number of finer radii is attained by the intercalation of 

 radiating striae, some of which reach even into* the umbonal region, between 

 the before mentioned prominent costae, while others set in a little above the 

 second bifurcation ; thus the marginal portion of the valve is covered by 

 numerous fine striae. 



The dorsal valve is covered by a similar system of striae, with the only 

 exception, that in this valve, between the first and second bifurcation of one 

 and the same radius, is only a short distance. In well preserved specimens 

 the entire surface of both valves is covered by fine concentric lines of growth, 

 some of which are more prominent, and divide the surface into concentric 

 zones. In most of the shells these concentric lines, with the exception of the 

 most marked ones, are obliterated by either silicification or exfoliation. Shells 

 from different localities show considerable variation in form and size. Our 

 shells, although differing in size, are uniform in shape and in the ornamenta- 

 tion of their surface. With very few exceptions, they have all a brownish-red 

 color, which is entirely superficial, not penetrating into the body of the shell 

 to any extent. 



Formation and Locality. Occurs rather abundantly in the upper strata of the Devonian forma- 

 tion, in Jefferson county, Ky., and in Clark county, Indiana. Verj- fine and well preserved specimens are 

 found, lacking only the fine concentric lines of growth. The illustrations on plate 18 represent the differ- 

 ent sizes of the shell as found in our rocks, and figure 22, plate 33, a very young specimen. 



Strophodonta hemispherica. HALL. 



Plate XVIII., figures 4, 5 and 6. 



Strophomena (Strophodonta) hemispherica, Hall. Tenth Keport on St. Cab., p. 113 1857. 

 Strophodonta hemispherica, Hall. Pal. N. Y., Vol. 4, p. 901867. 



Shell of large size, sub-hemispheric, semi- elliptical in outline ; hinge -line 

 equal or greater than maximum width of shell; cardinal extremities always 



