50 PALAEONTOLOGY OF KENTUCKY. 



and in others the length is the larger. Hinge-line is also very variable ; in 

 some specimens it is scarcely noticeable, in others of considerable size. Ven- 

 tral valve regularly convex in young individuals, but becoming gibbous in old 

 ones. The mesial sinus is regularly developed, but undefined and very shal- 

 low, even at the front ; it starts below the umbo and forms a slight convexity 

 in the basal margin, which is in most shells regularly curved ; beak elevated 

 and strongly incurved over the fissure, which it partly, sometimes wholly, 

 closed ; the foramen almost as wide in the base as its height ; cardinal area 

 sometimes scarcely visible, while again in some shells it forms a regular tri- 

 angle; bounded by a faint but distinct elevation on each margin. Dorsal valve 

 in young shells more or less convex, and sometimes gibbous in the upper part, 

 and often only moderately convex in older shells ; mesial fold starts below 

 the umbo, never well-defined, and not much elevated, still distinctly developed. 

 Surface ornamented by rounded or sometimes- by somewhat angular plications, 

 which, in rare instances, reach all the way back to the apex, but which are 

 generally limited to the lower half or two-thirds of the valve ; they increase 

 by bifurcation. The interior of the ventral valve has an elongate spoon- shaped 

 pit ; the inner extremity of which is free for a considerable extent, and the 

 upper part supported on a central septum, which usually extends less than 

 half the length of the shell from the apex. In the dorsal valve, the crura or 

 lamellae are conjoined at their bases, making a Y-shaped trough or pit, which 

 is attached to the valve in its upper part and continues sessile for about one- 

 half the length of the shell. This species shows much variety in aspect and 

 form, caused by age and the conditions under which it lived, which were either 

 favorable or unfavorable to its development. Conrad's species, Atrypa octo- 

 costata, includes forms which are easily connected by intermediates with Pent, 

 arata, and which, therefore, belong to this species. In size it differs greatly. 

 The figures on plate XIII. represent an individual of more than average 

 size. 



Formation and Locality. Occurs, not rarely, in the Corniferous limestone of Kentucky and Indi- 

 ana, at and around the Falls of the Ohio, where fair and sometimes even very fine specimens are procured. 



Fentamerella papilionensis. HALL. 



Pentamerus papilionensis, Hall. Geol. Hep. of Iowa, Vol. I., part II., cited on page 514 1860. 

 Pentamerus papilionensis, Hall. Thirteenth Rep. on the State Cabinet, p. 86 1860. 

 Pentamerella papilionensis, Hall. Pal. N. Y., Vol. IV., p. 377 1867. 



Shell of medium size, ventricose, broadly ovate, often wider than long, more 

 or less gibbous and arcuate in old shells. 



Ventral valve gibbous or ventricose above, becoming depressed in the middle 

 into a broad, shallow, undefined sinus, which scarcely reaches to the beak, and 



