NIAGAEA SROUP. 261 



extension laterally. The depression on the dorsal valve, which is indicative of Spirifer, is also 

 shown in the ventral valve, and there is usually a slight indentation in front. 



This species is exceedingly like one in the Delthyris shaly limestone, though the latter is a 

 stronger shell, and has usually a deeper indentation in front. 



Fig. 1 a, b. Young specimens of this species. 



Fig. 1 c. The ventral valve of a full grown individual, showing the beak of the dorsal valve. 



Fig. 1 d. Profile view of the same, the shell slightly compressed. 



Fig. 1 e. View of the beaks of the same. 



Position and locality. In the shale at Wolcott, the only locality at which it has been 

 observed. 



613. 26. ORTHIS PYRAMID ALIS («. sp.). 



PL. LII. Fig. 2 o-i. 



Subpyramidal, usually minute ; vertical valve flat, semicircular, with a depression along the 

 centre ; dorsal valve extremely elevated at the beak ; area large, triangular, extending to the 

 extreme angles of the cardinal line ; plane of the area vertical to the plane of the ventral 

 valve ; surface of shell marked by strong diverging straight striae, which, on the cardinal 

 margins, scarcely reach the beak ; striae sometimes dichotomous, and crossed by concentric 

 lamellose striae. 



This peculiar little species is readily recognized by the pyramidal form and large triangular 

 area of the dorsal valve. The species bears considerable similarity to the 0. disparilis of the 

 lower Silurian strata, but it is less extended laterally, is proportionally longer, and the beak of 

 the dorsal valve much more elevated. The number of plications in old shells of this species is 

 about the same as in 0. disparilis, but they are crossed by more elevated striae. In the young 

 shells the plications are often dichotomous, though not uniformly so. 



Fig. 2 a, b, c, d. Dorsal, ventral, cardinal and profile views of a specimen of the ordinary size. 

 Fig. 2 e, f, g, h. Similar views of a much larger shell. The usual size is that of the preceding 



figures of which many specimens have been found, while but the single large one 



figured has been found. 

 Fig. 2 L An enlargement of the surface, showing the sharp plications crossed by concentric strije. 



Position and locality. In the decomposing shale at Lockport. 



