DOSSILS OF THE SlLURlAft AND DEVONIAN BOOKS. 61 



impossible to describe minutely every possible form of this species ; I shall, 

 therefore, limit myself here to the description of the forms occurring in the 

 vicinity of Louisville, which are represented on plate XXXIII. Shell of 

 medium size, longitudinally sub-elliptical or sub-quadrate ; somewhat gibbous 

 in the upper portion, but depressed-convex towards the front ; trilobate, the 

 middle lobe largely extending ; beak of the ventral valve elevated above and 

 incurved upon the umbo of the dorsal valve ; both valves almost equally con- 

 vex and their surface marked by concentric lines of growth, of which some are 

 very fine and scarcely noticeable, others, however, very strong and dividing the 

 surface into different zones. The beak of the dorsal valve incurves into the 

 other valve below its beak. All the specimens in my cabinet are casts ; some 

 of them show undoubtedly, though only faintly, traces of longitudinal striae 

 on the middle lobe of both valves. 



An individual of average size has the following dimensions : length, two 

 inches and one-quarter ; width, one inch and three-fourths, and thickness about 

 one inch. 



Formation and Locality. Found in great abundance in the lower strata of the Niagara and in the 

 Clinton group, in Jefferson county, Ky., eastward from the city of Louisville. Though fractional shells of 

 this species are numerous, well preserved or even fair specimens are rare. 



Pentamerus oblongus, var. cylindricus. HALL AND WHITFIELD. 



Plate XXX., figures 2, 3 and 4. 



Pent, oblongus, var. cylindricus, H. and W. 24th Kep. N. Y. State Museum, pi. 183 1872. 

 Pent, oblongus, var. cylindricus, H. and W. 27th N. Y. State Museum, pi. 10, figs. 13 and 141876. 



Shell of medium size, sub- cylindrical in form; length exceeding the width 

 almost three times; very gibbous in the upper portion, gradually decreasing 

 towards the front. Ventral valve . a little more convex than the other; beak 

 elevated above and arched over the umbo of the other valve; greatest convexity 

 a little above the middle, from where it curves regularly towards beak and 

 front, but sloping abruptly, almost in a straight line, to the lateral margins. 

 The trilobate character of Pent, oblongus is very slightly noticeable. Dorsal 

 valve less convex and more regularly curved gently to beak and front, more 

 rapidly to the lateral margins ; beak incurved into the fissure of the other 

 valve below its beak. Surface of both valves is divided into different zones by 

 strong concentric lines of growth. The specimen figured measures in length 

 three inches ; in width one inch and three-eighths ; and in thickness a little 

 less than the width. 



Formation and Locality. Found in the Niagara and Clinton groups in Jefferson county, Ky., east 

 of the city of Louisville ; somewhat rare. 



