42 PALAEONTOLOGY OF KENTUCKY. 



base, and increases rapidly in depth and width towards front ; it has a lingui 

 form basal extension, which, however, does not deflect from the regular curve 

 of the sinus, to meet the dorsal indentation. Ventral umbo is more or less 

 prominent in different specimens, the beak elevated, pointed and arched, but 

 not incurved ; hinge-area is short but high, forming in some individuals almost 

 an equilateral triangle ; the larger portion of the hinge-area is occupied by a 

 comparatively large, open, triangular fissure. The dorsal valve is varying 

 from moderately convex to gibbous, having its greatest convexity just below 

 the umbo, from where it slopes in a more or less stronger, regular curve to 

 the baso-lateral margins, and abruptly to the apex and the cardinal borders ; 

 it has a mesial fold of moderate elevation, which commences to rise above the 

 general surface just below the umbo, from where it extends to the base, only 

 slightly increasing in width and depth. The dorsal umbo is somewhat in- 

 flated, and the beak small and incurved. The dorsal hinge-area is small, 

 about one-half the size of the ventral. The surface is covered by simple, 

 rounded or sub-angular radiating plications, of which usually three, but 

 sometimes four, occupy the sinus, and generally four, but sometimes five, are 

 placed on the mesial fold, while from four to seven or eight are seen on each 

 side of the mesial fold or sinus. In some specimens of this species there 

 exists a decided inequality in the lateral parts, consisting not so much in size 

 as in the number of plications. I have, for instance, one shell before me, 

 where the right side of the dorsal valve has only four ribs, while the left side 

 contains six. On the ventral valve is the same difference between the number 

 of the ribs of both sides. This inequality exists in a good many, though not 

 in all specimens. There is an indication of very fine radiating lines on and 

 between the plications. Fine concentric lines cross the plications, which 

 become lamellose and imbricating towards the front. 



Formation and Locality. This species is named in honor of W. M. Linney, the able Assistant 

 Geologist of our State, who collected this shell in the shales of the Hudson River or Cincinnati group, 

 near Danville, Kentucky. It is found in excellent, almost perfect specimens, and seems to be somewhat 

 abundant It also occurs in the base of the Lower Hudson, in Fayette and Franklin counties, Kentucky. 



Orthis nisis. HALL. 



Plata XXVII., figures 4 and 5, 



Orthis nisis, Hall. 24th Rep. on N. Y. State Museum, page 181 1872. 

 Orthis nisis, Hall. 27th Rep. on N. Y. State Museum, pi. 9., figs. 1-8 1875. 



Shell depressed-pyramidal when resting on the dorsal valve. Dorsal valve 

 semi-elliptical, flat or slightly concave in middle, and gently convex on each 

 side. Length and width about as two to three ; area about one-quarter as 

 wide as that of the ventral valve. Ventral valve depressed-pyramidal; the 



