74 PALAEONTOLOGY OF KENTUCKY. 



becomes depressed, which depression increases in depth, but more rapidly in 

 width, towards the base or front, which it occupies almost to the full extent of 

 basal length ; umbo is small, and the beak elevated and slightly incurved. The 

 dorsal valve is somewhat larger than the ventral, and also more convex ; it is 

 divided into two halves by a central depression, which extends f rom the beak 

 to a point below the middle of the valve, and which is most prominent in 

 umbonal region ; it is occupied by six single strong plications which reach 

 entirely back to apex. 



The two outside of these six plications form the lateral limit of the depres- 

 sion, which ceases a little below the middle of valve, where the said six 

 plications become elevated above surface, and all six rising to the same level 

 extend to the front, where they form a mesial fold, which is entirely flat on top ; 

 the lateral , portion, of the valve outside of this mesial combination of sinus 

 and fold, is regularly and gently curved from beak to the lateral and baso- 

 lateral margins, while it slopes abruptly towards the cardinal lines, along which, 

 in both valves, the borders are inflected, meeting under a very obtuse angle, 

 but with its apex outward ; these inflected borders are entirely smooth ; the 

 umbo is small, also the beak, which is incurved into the other valve. The 

 surface is ornamented by strong, rounded, simple, radiating plications, of 

 which the ventral valve has seven in its mesial sinus, and five on each side of 

 it ; while the dorsal valve has six on its mesial fold and six on each side of the 

 same. All these ribs reach back to the beak, and are of about equal strength, 

 with the exception of the two outside ones on each side of the mesial fold and 

 sinus, which are smaller, and extend only for some distance back into the shell, 

 becoming obliterated by the smooth cardinal borders. No other surface-mark- 

 ings are observable. The specimen here described measures : length, six lines ; 

 width, nine lines ; depth, three lines. This is one of the most beautiful Rhyn- 

 chonella, as far as its shape is concerned, and for that reason I have named it 

 bellaforma. I do not know any species of its congeners to compare it with, 

 except, probably, Rhyn. tennesseensis of Roemer, but not with that form of 

 said species which Prof. Hall has, in late years, placed in the species of Rhyn. 

 stricklandi. To Rhyn. tennesseensis it has some resemblance, but differs from 

 it by its small depth, its being more transverse, its smaller and more numerous 

 plications, its cardinal borders, which are not as much inflected as in that 

 species ; in our shell the marginal angle at said borders has its apex outward, 

 while the same angle in Rhyn. tennesseensis_ has its apex inward. But the 

 most characteristic feature of our shell is its peculiar combination of mesial 

 sinus and fold on its dorsal valve, by which it is easily distinguished from any 

 other Rhynchonella. 



Formation and Locality. I found this interesting shell, finely preserved, in the Niagara clay, east 

 of our city. So far this one specimen only is known to me. 



