FOSSILS OF THE NIAGARA GEOUP. 139 



ties, and show no evidence of radiating striae or obscure plications, as is 

 sometimes the case on those just referred to. The species is readily dis- 

 tinguished from others associated with it by its small, round, or globose 

 form, and the trilobed front of the valves. It differs from the ordinary 

 forms of Pentamerus in having the middle of the dorsal valve elevated in 

 the form of a mesial fold, while in true Pentamerus it is depressed, form- 

 ing a sinus on this side, with a correspon^ding elevation on the larger or 

 ventral valve. 



Formation and locality : In limestones of the Niagara group, at Yellow Springs, Ohio. 

 Collected hy Prof. E. Orton. In collections of Columbia College and Prof. James 

 Hall, and Ohio State Collection. 



Pentamerus pergibbosus (n. sp.) 



Plate 7, figs. 10, 11. 



Shell of medium size, extremely and extravagantly gibbous ; propor- 

 tionally 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 not more than one-third as wide, while the depth of 

 the united valves more than equals the length of the dorsal valve. 

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

 prominent and narrower than the opposite. Ventral valve about twice 

 as deep as the dorsal ; most prominent and ventricose about one-third of 

 its length from the beak; somewhat gradually sloping from this point to 

 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 strong lines of growth ; beak wide, strongly incurved 

 to the foramen of the ventral valve. Surface of the internal casts (the 

 condition in which the specimens occur) strongly constricted by concen- 

 tric lines, marking stages of growth, but placed at irregular distances, 

 and often dividing the surface into several strongly marked transverse 

 lobes. 



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. 



