62 PALAEONTOLOGY OF KENTUCKY. 



PentamerilS pergibboSUS. HALL AKD WHITFIELD. 



Plate XXIX., figures 23 and 24. 

 Pentamerus pergibbosus, Hall and Whitfield. Pal. of Ohio, Vol. II., p. 139, pi. 7, figures 10 and 111875. 



Shell of medium size, extremely and extravagantly gibbous , proportionally 

 very elongate from beak to base, and very narrow ; the greatest width being 

 near the front, and equal to only about two-thirds the depth of the two valves 

 when united in the broader specimens, and in some extravagant cases even less, 

 while the depth of the united valves almost equals the length of the dorsal 

 valve. Beaks distant and strongly incurved, that of the ventral valve the most 

 prominent and narrower than the opposite. Ventral valve more than twice 

 as deep as the dorsal valve; most prominent at the umbo, from where it slopes 

 somewhat gradually towards the front margin; triangular foramen large, 

 higher than wide, partially filled by the beak of the opposite valve. Dorsal 

 valve more regularly arcuate than the ventral, almost evenly so, except for the 

 constrictions of the surface by lines of growth ; beak wide, strongly incurved 

 into the foramen of the other valve. Surface of the internal casts, in which 

 condition the specimens occur, strongly constricted by concentric lines of 

 growth, which are placed at irregular distances, and often dividing the surface 

 into several strongly marked transverse zones. The interior of the shell has 

 been characterized by large longitudinal septa, as seen by the cavities left in 

 the casts ; that of the ventral valve extending fully to or beyond the middle of 

 its length, while those of the dorsal valve reach about two-thirds of the length 

 of the valve, in all the specimens examined, and in one example almost to the 

 front margin, and vertically to the entire depth of the valve. The surface of 

 the shell, in its original condition, has been marked by fine radiating striae, 

 which are still visible on the casts of some individuals near the front of the 

 valves, although the most of them appear to be smooth except for the concen- 

 tric constriction. This species is most nearly related to Pent, occidentalis, Hall, 

 from the Onondago salt group, Gait, Canada West; it differs, however, very 

 materially from that one in the much greater depth of the dorsal valve, and also 

 in the relative thickness of the longitudinal septa, that one having them very 

 thick and strong; also in the finer striation of the surface. From Pent, 

 littoni, another closely allied species, it differs in being more extravagantly 

 gibbous, and narrower, and in being more finely striated. Copied exactly from 

 Hall & Whitfield' s description in the Ohio Report, Vol. II. 



Good specimens in my possession show also faint radiating striae in the front, 

 still I doubt that the original shell was covered with radii. I have, for in- 

 stance, some specimens of Spir. radiata, which, on the larger portion of the 

 surface, retain the shell, while the front part is exfoliated. The shell-covered 

 portion shows no plications nor traces of them, while the exfoliated front is 



