54 PALAEONTOLOGY OF KENTUCKY. 



in a slightly curved, almost straight line to the front ; towards the lateral 

 margins it slopes in a gentle, regular curve, but approaching the margins, it 

 turns rapidly towards the dorsal valve, where it forms a smooth border ; the 

 beak is small, straight, and very little elevated above that of the dorsal valve, 

 often touching the same at equal height. The dorsal valve is depressed convex, 

 maximum convexity below the umbo, from where it slopes in a very gentle 

 but regular curve to front and sides ; only a very small strip of this valve at 

 the lateral margins is rapidly and abruptly bent, to meet the smooth marginal 

 border of the ventral valve ; the umbonal part is strongly curved towards the 

 other half of the shell ; the beak strongly incurved beneath or against the other 

 beak. 



Surface is marked by fine radiating striae, single and rounded, and number- 

 ing from thirty to forty. The surface is generally divided by three promi- 

 nent lines of growth into four zones, which are easily noticed on any fair speci- 

 men. The umbonal zone is always smooth, either the striae did not develop 

 before a certain age, or if this portion was ever covered with radii, they became 

 obliterated in the course of time. 



Formation and Locality. This beautiful species, of which my own cabinet possesses a few very fine 

 and perfect specimens, is found in the Niagara rocks in the quarries east of Louisville, Ky. It is very 

 rare. 



Pentamerus globulosus. K.SP. 



Shell very small ; sub-globose ; width exceeding the length ; very ventricose 

 or gibbous ; ventral valve very convex ; greatest convexity about the middle 

 of the valve, from where it slopes, in a regular but strong curve, to the beak 

 and to the lateral and basal margins ; to the cardinal lines it slopes more 

 abruptly, joining with the curved margins of the hinge-area ; the cardinal area 

 is small and not defined in its margins, which, as before stated, are curved and 

 coalesce with the surface of the valve ; umbo is very prominent ; the beak is 

 elevated and strongly incurved, but does not touch the beak of the opposite 

 valve, from which it remains sufficiently distant to show a moderately sized, 

 open, triangular foramen in the ventral hinge-area. 



Dorsal valve is sometimes as convex as the ventral, though usually it is less 

 so. It is most convex in the umbonal region, but flattens in the lateral parts. 

 Below the middle of the valve its central part becomes depressed into a shal- 

 low sinus, which does not reach beyond the basal third of the valve ; it is most 

 perceptible in the basal margin, outside of which it is scarcely to be noticed. 

 Corresponding with this mesial sinus the ventral valve shows a mesial elevation, 

 which is also confined to the basal third of the valve, and outside of the front 

 margin hardly observable. In some specimens these mesial depressions and 



