FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 63 



plainly plicate, showing that striae or radii may have been obliterated from the 

 surface of the shell, even when internal casts bear undoubted evidence of their 

 existence. Pent, pergibbosus, as found in our quarries, a specimen of which is 

 copied on plate XXIX., can not be confounded with other species ; it must be 

 recognized at the first glance. 



Formation and Locality. Found in the middle strata of the Niagara rocks, in our quarries east of 

 the city of Louisville, where it is, however, exceedingly rare. I am not aware that, outside of the speci- 

 mens in my cabinet, and those formerly owned by Rev. H. Herzer, but afterwards acquired by Prof. Hall, 

 of Albany, N. Y., any others are in existence. 



Pentamerus uniplicatus. N. SB. 



Plate XXXIII. .figures 25 and 20. 



Shell below medium size ; sub-triangular ; width exceeding the length ; 

 greatest width below middle of shell. 



Ventral valve very convex or gibbous, having its greatest convexity a little 

 above middle of valve, from where it slopes in a moderate but regular curve to 

 the beak, and to the baso-lateral margins, but very abruptly to the cardinal 

 borders, which are broadly rounded; the baso-lateral margins are regularly 

 curved, and the front is slightly sinuate. The central portion of the ventral 

 valve is somewhat elevated, extending from beak to front, and forming a mesial 

 fold, which is narrow, and, in fact, not at all elevated above the general surface 

 of the valve ; its apparent elevation is due to two broad furrows, between which 

 it is situated. This mesial fold is flat on top, and has below the umbo a shallow 

 central depression. Umbo is prominent, and beak elevated and incurved. The 

 dorsal valve is moderately convex in the umbonal region, sloping from there 

 rapidly to the cardinal lines, near which it is slightly depressed ; towards the 

 baso-lateral margins dorsal valve becomes flat. The middle portion of the 

 valve from beak to front is depressed, forming a mesial sinus, which increases 

 in depth and width towards the front, and which is bounded on each side by 

 an obscurely marked plication ; this marginal rib is, however, only in the basal 

 half of the valve separated from the balance of the surface by a faint depres- 

 sion. The bottom of the mesial sinus is occupied by a low rounded plication, 

 which extends to umbo. The umbo is small, and the beak pointed and in- 

 curved. Outside of the ribs already mentioned, and some faint concentric lines 

 of growth, there are no other surface-markings indicated. Size of shell is 

 shown in illustration. 



Formation and Locality. The specimen here described and figured is the only one so far known to 

 me. I found it in the Niagara rocks east of the city of Louisville, Ky. 



