FOSSILS OF THE NIAGARA GROUP. 137 



Genus PENTAMEEUS, Sowerby. 

 Pentameeus oblongus, Sowerby. 



Plate 7, fig. 9. 



Pentamerm oblongus, Sow. ; In Murch. Silurian Syst., p. 641, pi. 19, fig. 10. 



Pentamerus oblongus, Hall; Pal. N. Y., Vol. II., p. 79, pis. 25 and 26. 



Pentamerus bisinuatus, McChesney ; New Pal. Foss., p. S5, 1859 ; also re-issue in Trans. 



Acad. Nat. Sci., Chicago, Vol. I., pi. 9, fig. 1. 

 Pentamerus oblongus, Authors. 



Shell large, elongate ovate in outline, broadest below the center, often 

 lobed, and not unfrequently indistinctly plicated in the forward portion. 

 Valves depressed convex in the younger and medium sized specimens, 

 becoming ventricose, gibbous, or even inflated, in older or larger indi- 

 viduals ; the sides of the valves in the upper portion, but below the car- 

 dinal margins, flattened or concave, sometimes forming a shallow groove 

 extending from below the beaks of each valve and reaching nearly to the 

 widest, or largest, part of the shell. This feature is not seen during the 

 very young stages of growth. Front margin of the valves strongly con- 

 stricted or lobed by depressions of the surface, which extend from the 

 margin to a greater or less distance toward the rostral portion of the 

 shell on each valve. Ventral valve much deeper than the dorsal, 

 especially in the more ventricose forms, with a strong, projecting, mod- 

 erately incurved beak, beneath which is a broad, triangular fissure; lat- 

 eral margins of the beak sub-angular. Dorsal valve less ventricose, 

 most prominent on the upper third of the length; beak small, closely 

 incurved. 



Surface of the valves smooth or distinctly plicated, and marked by 

 frequent, strong, concentric lines of growth at irregular distances. 



The internal casts of this species, the condition in which they are 

 usually found in Ohio, has the beak of the larger valve strongly and 

 deeply divided down the center, to about one-third of the entire length, 

 by the removal of the longitudinal septum, which has supported the 

 broad, spoon-shaped process beneath the beak, the filling of which is 

 seen occupying the space between this portion of the valves. The 

 dorsal valve is also divided longitudinally by two slits, caused by the re- 

 moval of the septa that has characterized this valve. These slits fre- 

 quently extend nearly, or quite, to the middle of the valve, but are 

 placed very near to each other, diverging very gradually as they recede 

 from the beak. 



