136 Fossils of the Niagara and Clinton Gronps, 



perplexity to amateui-s, and can only be well understood by collecting and 

 observing frequently numerous specimens, thus familiarizing the eye with its 

 appearance, and ascertaining its gradations of form. It occurs in the Clintort 

 group, and less commonly in the Niagara shale. 



Fig. 4. Atrypa congesta, (Conrad,) Clinton Group. — This species is 

 nearly globular, or ovate, with a deep sinus which commences at the beak. — 

 The shells are in most instances smooth, or only marked by concentric stria'.. 

 On each side of the sinus and mesial fold there is an additional fold, well 

 shewn in the figure. The beak of the ventral valve is strongly incurved, or 

 hooked over the dorsal valve. Professor Hall says " it is readily recognised 

 in its usual appearance by its rotund and gibbous form. The variations are 

 mainly due to the greater development of the caringe on either side of the 

 mesial fold and depression, which sometimes give the shell a different character, 

 having three prominent folds on the dorsal and four on the ventral valve. — 

 This change usually takes place in the older individuals, while the younger 

 ones present only the mesial fold and depression. The specimens usually 

 found are smooth, the striae ha'^nng been worn or dissolved away ; but in 

 perfect specimens they appear as fine raised thread-like lines." 



The species is found in the Clinton group. One of the localities given 

 by Professor Hall, is Flamborough Head, Canada West. Congesta, con- 

 sisting of heaps. 



Figs. 5 and 5, [Orthis elegantula) Clinton and Niagara Groups. — 

 This species much resembles Orthis testudinaria of the Lower Silurian 

 rocks. The dorsal valve is nearly or quite flat. The ventral valve with a 

 highly elevated beak curving over the area. The surface is covered with 

 fine striae. The size is about that of the figures. 



It occurs in all the localities of the Niagara shale, and also in the Clin- 

 ton group, and is abundant in Europe. Like its near relation 0. testidunaria, 

 it therefore has a wide geographical distribution. 



Fig. 6, [Orthis fiahelluhm,) Niagara Group. — This fossil is thas 

 described by Professor Hall : — " Shell semioval, hinge-line equal to the 

 width of the shell, surface marked by twenty-four to thirty simple rounded 

 plications, which are equal to the space between them, plications usually 

 smooth, with the remains of concentric striae crossing the depressions between, 

 and rarely appearing on the elevations, a few strong imbricating lines of 

 growth near the base, cardinal area usually narrow, and extending to the 

 extremity of the hinge line." 



" It is usually found so much flattened that the two valves appear to be 

 equal. The ventral valve in perfect specimens is more convex than the 

 dorsal. The foramen is broadly triangular with a thin sharp tooth in the 

 centre and a stronger one on each side projecting into the centre, the muscular 

 impression has a strong rounded ridge down the centre with a depression on 

 each side, but the margins are not well defined. The interplication on the 

 Inside appear to be duplicate, or have a groove along the centre. In some 

 si>ecimens the plications on the interior extend but half-way to the beak ; 

 while in others, that are apparently of the same species, they extend to tht? 

 muscular impression." 



