72 PALAEONTOLOGY OF KENTUCKY. 



It appears that in the umbonal half of the shell only two plications existed 

 before they became obliterated, and that by bifurcation the four ribs were 

 produced, which now occupy sinus and fold. Some individuals have five ribs 

 instead of four. The lateral parts of this shell are often covered with several 

 faintly visible plications, but in most of the specimens these lateral ribs become 

 entirely obsolete, and the surface outside of sinus and fold is free from any 

 markings whatever. 



Specimens of this species are seldom in a fair condition ; they are found in 

 the Genesee shale, and their shell being very thin, they become generally dis- 

 torted and compressed. The inclosing shale or slate did not possess sufficient 

 density and hardness to resist the pressure of the superimposed rocks, but 

 became compressed itself, and thus the imbedded shells suffered the same fate 

 as their matrix. I have found in the shales at the Falls of the Ohio, on the 

 Kentucky shore of the river, an almost perfect specimen of this species, with 

 a very slight distortion, and on the Indiana side I picked up another shell 

 belonging to L. quadricostata, not distorted at all, but a little defective at the 

 front, which, however, was easily restored ; and thus I possess two specimens 

 which show exactly the form and surf ace- markings of this species. Prof. 

 Hall states in his description of this shell, in Pal. N. Y., Vol. IV., page 358, 

 that on account of the compressed and crushed condition of the fossil, its real 

 form and its correct proportions could not be determined. The specimen before 

 me measures nine lines in length, nine and one-half lines in width, and five and 

 one-half lines in depth. Another specimen found at Lexington, Indiana, 

 measures thirteen lines in length, one inch in width, and seven to eight lines in 

 depth. These dimensions show that, in most instances, this shell has its width 

 and length about equal, while, its convexity may vary somewhat ; its greatest 

 width is either at the middle of the shell or a little below it. 



Formation and Locality. Found mainly in the Genesee shales, being the top strata of the Devo- 

 nian formation, at the Falls of the Ohio, in Kentucky, and at Lexington, Indiana, but some specimens are 

 also found in the rotten hornstone just below said shales. 



Genus Rhynchonella. 



Fischer. 



Rhynchonella, Fischer. Mem. Soc. Imp., Mosc. 1809. 

 Etymology : rhynchos, a beak ; ella, little. 



Shells trigonal, acutely beaked, usually plicated, dorsal valve elevated in 

 front, depressed at the sides. Ventral valve flattened or hollowed along the 

 center; hinge-plate supporting two slender curved lamellae; dental plates 

 diverging. 



The foramen is at first only an angular notch in hinge-line of the ventral 



