FOSSILS OF THE SILUEIAN AND DEVONIAN ROCKS. 57 



its margins, as it curves into the general surface of the valve ; the triangular 

 fissure is either open from cardinal line to apex, or partly closed by beaks of 

 both valves. Dorsal valve less convex than ventral ; greatest convexity at the 

 umbo, sloping from there in a gentle curve to the sides and front, but more 

 rapidly to the cardinal line ; about the middle of the valve a depression sets 

 in, extending in width over three plications; it increases in depth but very 

 slightly up to the front, where it forms a small basal extension, which 

 deflects downwards to meet the small indentation of the other valve. In some 

 specimens no basal extension exists, or is so small as not to be noticed ; umbo 

 prominent, and beak strongly incurved below that of the other valve into the 

 fissure of the latter. Surface marked by from ten to twelve rounded or sub- 

 angular plications, which are plainly visible at the lateral and basal margins, 

 but become obsolete before reaching the umbo on both valves. The specimen 

 illustrated represents about the largest size which this species ever attains. 



Formation and Locality. Occurs in the upper strata of the Niagara rocks in the quarries east of 

 the city of Louisville, Ky., where it is not very rare. I name this species in honor of W. T. Knott, of 

 Lebanon, Ky., the efficient Assistant Geologist of our State, who made us acquainted with the geology of 

 his district, 



Pentamerus knighti. SOWSKBY. 



Plate XXIX., figures 1, 2 and 17. 

 Pentamerus knighti, Sowerby. Min. Couch., Vol. I 1812. 



Shell of medium size, sub-ovate and very gibbous ; narrow, with its greatest 

 width at the base, which is regularly carved. Depth about four-fifths of its 

 length. Surface plicated. 



Ventral valve about twice as deep as the other, very gibbous, and regularly 

 arched from beak to front. From the longitudinal center line on the summit 

 of the valve it curves for a certain distance, about half way, gently and regu- 

 larly towards the lateral margins, then it slopes very abruptly towards the 

 cardino-lateral angle, where it forms on each side of the beak a large concave, 

 smooth field. The umbo is narrow but very prominent ; the beak strongly 

 arched over the umbo of the other valve, but not touching it. Dorsal valve 

 less gibbous than the other valve, regularly curved from beak to front, and also 

 towards the lateral margins, except in a small strip at those margins, where the 

 valve deflects abruptly downwards, joining the smooth concave field of the other 

 valve by a similar but considerably smaller field. This valve is wider than the 

 other, except at the front, where both are of equal width. The whole surface, 

 with the exception of the above mentioned concave fields in the cardino-lateral 

 angles, is covered by strong, simple sub-angular plications, reaching from beak 

 to front, and increasing as the shell grows in strength or size and in distance, 



GEOL. SUR. 8 , 



