FOSSILS OF THE SILURIAN AND DEVONIAN ROCKS. 49 



larger and more elevated at the beak than the dorsal valve, with a deep mesial 

 furrow, which commences at the beak and gradually deepens and widens 

 towards the base or front of the valve, where it forms a linguiform basal ex- 

 tension, which is strongly elevated to fit into a corresponding indentation of 

 the dorsal valve. The mesial depression of the ventral valve is margined by 

 a more or less obtusely carinated fold, produced by a depression of the shell on 

 each side of the sinus. Dorsal valve with a strong, elevated mesial fold, and 

 a depression on each side, beyond which is sometimes an obscure fold. Beak 

 of the ventral valve elevated and strongly incurved over beak of opposite 

 valve, while the dorsal beak is small and incurved. The entire surface is 

 covered, in well preserved specimens, with fine concentric striae, and with a 

 few imbricating lines of growth near the front. The specimen before me does 

 not show any of these lines on account of its silicified condition. This species 

 is subject to considerable variations in its form, but is, in spite of that, easily 

 identified by its rotund and gibbous form. 



Formation and Locality. This species usually occurs in the lower strata of the Clinton group; 

 but I found my specimen in the Niagara rocks near our city, from which it appears that the lower strata 

 of our Upper Silurian rocks must belong to that group, as is also indicated by several other fossils of de- 

 cidedly Clinton character. 



Genus Pentamerella. Han. 



Pentamerella, Hall. Pal. N. Y., Vol. IV., p. 3731867. 

 Etymology : Diminutive of pentamerus. 



Shells ovate, more or less rotund, with a sinus on the ventral valve, and a 

 mesial fold on the dorsal valve ; internal structure of the ventral valve as in 

 Pentamerus knighti. Dorsal valve with the crura or lamellae of the hinge- 

 plate conjoined, so as to form a separate, trough-shaped cavity, which unites 

 with the inner surface of the valve ; a narrow area on each side of the fissure, 

 and a flattened space or a false area along the cardinal margin of the valve. 

 Pentamerella arata is the type of this genus. 



Pentamerella arata. CONRAD. 



Plate XIII., figures 17 to 20. 



Atrypa arata, Conrad. Ann. Rep. on Pal. of N. Y., p. 55 1851. 

 Atrypa octocostata, Id. Ib. 



Pentamerus aratus, Hall. Tenth Eep. St. Cab., p. 1201857. 

 Pentamerella arata, Hall. Pal. N. Y., Vol. IV., p. 3751867. 



Shell of medium size, ovate, more or less convex or gibbous, becoming 

 arcuate-ovoid in old shells ; proportion of length to width variable ; in most 

 cases both are equal, but in some individuals the width exceeds the length, 



QEOL. SUR. 7 



