NIAGAKA OEOtTP. 



253 



Fig. 3 p. The interior of a similar shell, showing a slight variation in character. 

 Fig. 3 r. Cardinal view of the same, showing the dental lamella, and the callosity at the apex 

 of the foramen. 



Position and locality. This species is found in all localities of the Niagara shale, from 

 Wolcott in Wayne county, to the Niagara river. The species is eminently characteristic of the 

 Upper Silurian period, occurring in the same position in Europe as it does over wide areas in 

 the U. States. 



614. 27. ORTHIS HYBRIDA. 



PL. LIl. Fig. 4 a - 0. 



Orthis hybrida. Sowerby : Murohison, Sil. System, 1839, pag. 630, pi. 13, fig. 11. 

 — — Hall, Geol. Rep. 4th District, 1843, pag. 107 ; fig. 7, p. 105. 



Lenticular or depressed spheroidal, wider than long ; valves nearly equal, the dorsal one 

 depressed from the centre to the base, while the ventral valve is regularly convex, sometimes 

 with a slight depression in the centre near the beak ; beaks almost precisely equally elevated, 

 the dorsal one being scarcely incurved ; hinge-line much shorter than the width of the shell ; 

 surface marked by thin sharp dichotomous striae, which are arched upwards upon the sides and 

 hinge-margin ; radiating striae crossed by extremely fine concentric striae, which are visible 

 under a magnifier. 



This species is readily distinguished by the near equality of the valves, one of which has a 

 broad undefined depression along the centre. The surface characters are very similar to the 

 preceding, but the form of the shell, or of the separated valves, is quite sufficient to distinguish 

 it. The shell is rarely larger than the specimens figured, the prevailing size being about the same 

 as those figured by Murchison. 



Fig. 4 a, b, c. Figures of specimens showing three gradations of size. 



Fig. 4 d, e. Cardinal and profile views of fig. 4 c, which is compressed, but still exhibits the 



usual proportions of specimens. 

 Fig. 4 /. A specimen having the valves more convex than usual. 



Fig. 4 g. Profile view of the same specimen, showing the nearly equal convexity of the valves. 

 Fig. 4 A. A cardinal view of a specimen, showing the convexity of the dorsal valve, while the 



ventral valve is depressed in the centre. The dorsal valve is represented too extremely 



convex in this figure. 

 Fig. 4 i, k. The interior of the dorsal valve of this species, showing the muscular impression 



divided by a longitudinal ridge. On each side of this ridge there are two lobes or 



depressions, which are marked in the outline of the impression. Two strong diverging 



teeth project from the inner angles of the foramen. 

 Fig. 4 I. An enlargement of the muscular impression of the preceding specimen. 

 Fig. 4 m. The interior of the ventral valve, with a scarcely defined muscular impression having 



a strong ridge, proceeding from the central process, down the centre. The lateral 



teeth were originally strong, but have been broken off in the specimen figured. 



