535 



Orthis. 



frequent intervals by elongate pits or openings. The radiating 

 stri^ are crossed by fine, indistinct concentric strias, and at 

 greater intervals by distinct lines or varices of growth. 



On the casts of the interior of the ventral valve the muscu- 

 lar impression is ovate in outline, the length equal to more 

 than two thirds that of the shell ; width two-thirds the length ; 

 bilobed by the callosity of the adductor muscle. The sides are 

 somewhat distinctly lobed in three divisions. 



This species most closely resembles Orthis circulus of this 

 formation, but the valves are inequal, the outline less circular, 

 the greatest width being below the base, and is constricted at 

 the base. The muscular impression is larger. 



Formation a7id locality. Clinton group, above fossil ore, 

 from McKee's ore bank, seven miles north- west of Lewistown, 

 Mifflin county; also, ore mine north of Blackleg creek, Orbi- 

 sonia, Huntingdon county, Pennsylvania. V a, 



Orthis subnodosa, Hall. Figure from Collett's Indiana 

 Eeport of 1881, page 286, plate 27, fig. 17, ventral 

 valve, showing the nodose or knotty character of 

 the plications or folds. — Niagara^ Vh, 



I, . 



Orthis subquadrata, Hall. Pal. N. Y. Vol. 1, 1847. 



Tre7h- 



G-toirCanak 



tonw[iSi R, River — Geol. Canada, 1863, p. 165, fig. 146 a^K 

 Trenton.— Psil. Ohio, Vol. 1, 1873, p. 94, plate 9, fig. 2 a, (5, on 

 the [plate an error), dorsal valve; 2 5, ventral; 2 <?, inside dor- 



7 



