82 PALAEONTOLOGY OF KENTUCKY. 



and increase in size and distance from beak to front. The ventral valve has 

 its mesial depression greatly produced in front, and abruptly turned upwards, 

 which extension fits exactly in a corresponding indentation in the base of the 

 dorsal valve. There are no radiating striae visible, but a few concentric lines 

 of growth may be noticed. The shells of this species were originally identified 

 with Rhyn. tennesseensis, a species described by Prof. P. Roemer, of Germany, 

 from the Silurian of Tennessee, and they are, therefore, generally known 

 under that name. A closer examination and comparison showed the former 

 identification as erroneous, and placed our shells in their present species. 



Formation and Locality. This species occurs in great abundance and in fine and well preserved 

 specimens in the Niagara rocks at Waldron and St. Paul, in Indiana; in the Niagara strata of Kentucky, 

 near to and east of the city of Louisville, this species is also found, but very rarely, and then in defective 

 and distorted specimens. The best individual, so far as I know, ever found in our quarries, is figured on 

 plate 27; it is considerable smaller than the Waldron shells, but has their exact form and markings. An- 

 other specimen which I found associated with the former, has the same shape in its outlines, but shows only 

 faint traces of the plications ; its surface appears almost smooth. 



Rhynchonella tenuistriata. N - BP - 



Plate XVII., figures 27, 28 and 29. 



Shell rather small, sub-triangular or sub-pentagonal ; cardinal line forms a 

 right-angle at the beak; its two sides, which are somewhat concave or in- 

 curved, slope down below the middle of the shell ; here they meet the lateral 

 margins, with which they form again an almost right-angle; lateral margins 

 short, about one-third the length of the shell, almost straight or very slightly 

 convex ; basal margin straight, with a slight concavity. 



Ventral valve less convex than the dorsal, with its greatest convexity at the 

 umbo, from which it slopes in almost straight lines to the lateral margins ; the 

 cardinal margins deflect abruptly upwards to meet the margin of the dorsal 

 valve in one and the same plane ; below the umbo the central portion becomes 

 depressed, which depression increases in depth and width towards the front, 

 where it occupies the ralve to the full extent of the basal margin. This mesial 

 sinus is rounded, its margins are not well defined, and its depth becomes only 

 somewhat prominent at or near the front ; the umbo is small, the beak elevated 

 above that of the other valve, and very little arched. The dorsal valve very 

 little convex, almost flat in the umbonal region and below it to the basal mar- 

 gin, where a part of the front is elevated into a mesial fold. On each side of 

 this mesial fold the valve slopes down very abruptly to the baso-lateral mar- 

 gins. The mesial fold is only observeable at or near the front ; the umbo is 

 inflated, and the beak small and incurved into the opposite valve. The surface 

 of both valves is covered by slender, sub-angular or rounded radii, of which 

 there are five or six on each side of the mesial fold and sinus ; the fold is 



