FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 7Y 



the more gibbous specimens flattened. Ventral valve with the beak pointed 

 and incurved, depressed-convex in the middle, and gradually becoming 

 depressed and sinuate in front, two or three of the plications included in the 

 sinus. Dorsal valve a little the more gibbous, somewhat flattened in front of 

 the umbo. Three or four of the plications run straight from beak to front and 

 form a mesial elevation, the lateral plications are curving outwards to the 

 lateral margins. Each valve marked by from nine to twelve rounded or sub- 

 angular plications, which sometimes obsolete toward the beak. The concentric 

 lines are very obscure. 



This species resembles the Rhyn. neglecta of the Niagara group of New 

 York, but it is larger and more robust, with stronger and more rounded plica- 

 tions. (Hall.) 



Formation and Locality. Found rarely in the Niagara strata in the quarries east of the city of 

 Louisville, Ky. Our specimens are smaller than the Waldron shells, and the plications on the Kentucky 

 individuals are less marked. They even become sometimes almost obsolete on the lateral slopes of the 

 Shell, and seldom extend as far back as the ribs on Waldron specimens. , 



Rhynchonellu, louisvillensis. K. SP. 



Plate XXXI., figures 1 to 4. 



Shell of less than medium size among the Rhynchonellidae ; longitudinally 

 sub -oval or sub-trigonal; length and width about equal, the latter rarely ex- 

 ceeding the former slightly ; both valves about equally convex. 



Ventral valve moderately convex ; mesial sinus beginning in front of the 

 umbo, is broad and flat, deepens at the base, and has a considerable quadri- 

 lateral extension, fitting in a corresponding indentation of the other valve ; it 

 contains five plications ; beak small and pointed, and only slightly arched. 

 Dorsal valve somewhat more convex than the other ; mesial fold starting below 

 the umbo, and becomes prominent at the front, and contains, like the sinus, 

 five plications ; beak small, narrow, and incurving into the other valve beneath 

 the ventral beak. 



Surface is ornamented by four or five rounded ribs on each side of the mesial 

 depression or elevation; those on the lateral slopes of the dorsal valve are 

 abruptly curving outwards and downwards. In regard to measurement, the 

 figures on plate 31 give the dimensions of an average-sized specimen. 



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

 Kentucky and Indiana, where it is, however, of rare occurrence. As far as my information goes, only 

 three individuals of this species are known. This species has some resemblance to Hall's Khyn. indianensis 

 of the Niagara group, but it is easily distinguished from that species, differing from it by its larger number 

 of plications in the sinus and on the fold, by the greater extension of its mesial fold, and also by its 

 greater convexity or gibbosity. 



