2.38 Fossils of the Niagara and Clinton Groups, 



Figs. 11 aud 12, Atrypa ncghcta, (Hall,) Niagara and Clintgs 

 'Groups. — Professor Hall says this shell is " ovoid or subpyrimidal, beaks 

 acute, shell gradually enlarging from the beaks to the base, which, in old 

 shells, is deeply sinuate ; dorsal valve more convex than the ventral valve, 

 surface marked by simple sharp plications, which are crossed by fine con- 

 centric strise, and sometimes by a few imbricating lines of gi'owth ; ventral 

 valve with a mesial sinus below the middle, and a corresponding fold upon 

 the dorsal valve." 



" In the young shells the valves are equal, aud there is neither sinus or 

 -elevation ; but as the shell advances in size, the sinus becomes conspicuous. 

 There are generally three, and sometimes four plications in the sinus, and four 

 or five elevated on the opposite valve. The plications usually appear as if 

 Fraooth, except near the base where there are some strong imbricating linea 

 of growth. It is a very common species, and sufficiently distinct in all its 

 phases to be readily recognized." 



It is found in all the localities of the shale of the Niagara Group, and 

 5t also occurs in the Clinton group. 



Fig. 13, [Atrypa cuneata,) Dalman. — The principal character of this 

 .species is its long triangular shape. The plications are ten or twelve, three 

 or four depressed on the ventral, aud elevated on the dorsal valve. The beak 

 of the ventral valve is nearly straight, and perforated at the extremity. — 

 This shell is somewhat variable in shape, aud the specimens are usually 

 iflattened and distorted. Cuneata, Latin, wedge-shaped. 



It occurs m the Niagara shale, and also in several countries in Europe 

 in the Silurian rocks. 



Figs. 14 aud 15, Leptena transversalis, (Dalman,) Niagara Group. — 

 This species is semi-circular, and the ventral valve very convex, w^hile the 

 -dorsal valve is equally concave. The hinge line is sometimes equal to and 

 often longer than the width of the shell, as in fig. 14. The surface is marked 

 hj a number of elevated radiating ridges, the intervals between which are 

 more finely striated. Professor McCoy says of the English specimens, that 

 they are more globose than Leptena sericia, and " distinguished externally 

 by the fewer aud more distant linear ridges, and the very much finer longi- 

 tudinal strise between the thread4ike ridges, and their being besides so faintly 

 impressed as to be, in almost all cases, invisible to the naked eye, or a lens 

 of low power. The interior of the dorsal valve shews well the long parallel 

 muscular impressions. Fig. 15 is a view of the hinge line, exhibiting the 

 elevation of the marginal ridges by which these impressions are bounded. — 

 This is one of the most abundant of fossils in the Upper Siluriau rocks of 

 America, and we have specimens in our possession from the Trenton Lime- 

 stone 'that much resemble it. In Europe it is also quite common. 



Figs. 16 and 17, Leptena subplana, (Conrad,) Niagara Group. — This 

 shell is " semi-elleptical, length and width nearly equal, hinge line extending 

 beyond the width of the shell. The surface is marked by prominent sharp 

 striaj, which frequently bifurcate before reaching the margin. The radiating 

 gtriae are crossed by strong concentric striae. The area (Fig. 17,) extends 



