&2 PALEONTOLOGY OF KENTUCKY. 



into two plications in front of the beak ; these mesial ribs are considerably 

 prolonged in front, producing a sub-quadrilateral extension ; beak is prominent 

 and incurved ; cardinal area large, extending to the extremities, and bounded 

 by a well-marked, regularly curved line of demarcation ; fissure of moderate 

 size, but partly closed by the beaks of both valves. 



Dorsal valve depressed convex, curved slightly in the upper half of the 

 valve ; lateral portions of lower half almost flat ; mesial fold formed by three 

 strong plications, which are united into one single elevation on the umbo, 

 where it is only faintly visible ; below the umbo the three mesial ribs separate 

 and extend to a little beyond the front, where they are considerably elevated ; 

 beak moderate and incurved into the foramen of the other valve ; cardinal 

 area only linear. Surface marked by about twelve sub-angular plications, of 

 which those in the mesial depression and elevation are considerably stronger 

 than those on the lateral slopes ; the lateral ribs on the dorsal valve are single 

 and of equal size; those on the ventral valve increase by bifurcation, and those 

 nearest to the mesial furrows appear to be stronger than the more lateral 

 ones. 



This species resembles Pent, arata and Pent, papilionensis, also Pent, dubia ; 

 from Pent, arata it differs by its elongate form, its less numerous plications, 

 and by its mesial ribs, which are here limited to three in the fold and two in 

 the sinus, and which are of about double size of the lateral ones ; while in 

 Pent, arata the mesial plications are larger in number and of almost equal size 

 with the balance. From Pent, papilionensis it differs in its elongate form, and 

 in the peculiar character of the mesial plications, which in P. papilionensis 

 have the same number, but do not differ from the lateral ones by size; and 

 from Pent, dubia it differs also in form ; while P. dubia has its greatest width 

 about the middle of the shell, this one has its maximum breadth nearer to the 

 front, at more than two-thirds from the apex; P. dubia has more plications 

 generally, and also more on the fold and in the sinus. 



Formation and Locality. Found in the Corniferous limestone surrounding the Falls of the Ohio 

 in Kentucky and Indiana. It is of rare occurrence here in the neighborhood of the falls ; only two indi- 

 viduals of this species are known ; they belong to my own cabinet. 



Genus Pentamerus. 



Pentamerua, Sowerby. Min. Couch., Vol. I. 1814. 

 Etymology: penta, five; meros, apartments. 



This genus was proposed by Sowerby in 1813 to include all the shells allied 

 to Pent, knighti, which he made the type of the genus Pentamerus. Dalman 

 objecting to the name, on the ground that the shell was not five-chambered, 

 proposed in its stead the name Gypidia, but this latter was never accepted. 



