Fossils of the Niagara and Clinton Groups. 137 



Tliis species occurs in the shale of the Niagara group. There is a 

 upecies of the same name in the Lower Silurian rocks of England, and Prof. 

 Hall considers these American specimens to constitute a variety of the 

 English species. Flabellulum, Latin, a " little fan." 



Fig. 7, Sinrifer sulcatus, (Hisinger,) Niagara Group. — " Shell sub- 

 triangular and gibbous, cardinal line more or less extended, often pointed at 

 the extremities, surface plicated, plications four to seven on each side of the 

 mesial fold and sinus, crossed by strong imbricating lamellae, and longitudi- 

 nally marked by fine striae which are interrupted by the edges of the lamellae, 

 mesial fold of the ventral valve very deep towards the base of the shell." 



This species is readily distinguished by its roughly lamellate surface. — 

 It is one of the most common forms in the Niagara Group. It is found also 

 in the Silurian rocks of Europe. Sulcatus, Latin, furrowed. 



Fig. 8, Spirifer Niagarensis, (Conrad,) Niagara Group. — The surface 

 of this shell is marked by from twenty to thirty rounded plications, and these 

 are also striated longitudinally by fine equal striae. The mesial elevation 

 consists of a single large fold, and the sinus of a corresponding depression. — 

 It is a large shell when full grown, and both valves are about equally convex. 

 The ventral valve has the beak elevated and incurved over the area, which 

 is of medium size. The hinge line is usually shorter than the shell, and the 

 ears are rounded. The young shells have only ten or twelve plications on 

 their surfaces. 



This fossil is typical of the Niagara Group, and is readily recognized 

 by its rounded plications, which are evenly striated in a longitudinal direc- 

 tion. It occurs in the shale, and rarely in the limestone of the group. 



Fig. 9, Atry-pa nitida, (Hall,) Niagara Group. — " Shell ovoid, with 

 the beaks more or less extended, surface smooth, or with fine concentric striae 

 and a few conspicuous lines of growth towards the base, and sometimes on 

 the middle of the shell, valves nearly equally convex, the beak of the ventral 

 valve being much elevated above, and incurving over the dorsal valve, the 

 ventral valve sometimes marked near the base by a longitudinal depression." 



This species is very abundant in the shale of the Niagara group, and 

 somewhat variable in form. It usually has a smooth surface. It is most 

 abundant at Lockport. Nitida, Latin, smooth. 



Fig. 10, [Atrijpa reticularis,) Lixne. — This fossil has a very great 

 vertical range, being found in many of the formations from the Clinton 

 upward to the Chemung group. It is nearly circular. The ventral valve is 

 much more convex than the dorsal. The surface is ornamented by from 24 

 to 30 small rounded plications which bifurcate about one-third of the distance 

 from the beak to the margin, these plications are crossed by concentric 

 elevated lamellae which give to the surface a reticulated appearance, whence 

 the specific name reticularis, net-like, or reticulated. The dorsal valve has 

 often a shallow depression or sinus at the base, and the other a correspondiog 

 elevation. The beak of the dorsal valve is small, and but slightly elevated 

 above the ventral valve or hinge line. The cardinal angles are sometimes a 

 iittle extended beyond the width of the shelh 



