60 BRAGHIOPODS. 



Orthis tricenaria CONRAD. 



Plate xxxix, fig. 4. 



Orthis tricenaria Conrad, 1843: Proc. Acad. Nat. Sci., Phila., p. 333. 

 Orthis tricenaria Hall, 1847: Pal. N. Y. , vol. I, p. 121. 



Shell rather below medium [size, flattened, sides straight, 

 hinge-line as long as greatest width. Surface marked by strong, 

 rather large radiating costse, which curve slightly outward as 

 they leave the beak. 



Horizon and localities. Lower Silurian, Hudson shales : 

 McOune station ( Pike county ). 



Orthis missouriensis SHUMARD. 



Orthis missouriensis Shumard, 1855: Geol. Sur. Missouri, Ann. Kep., p. 

 205, pi. C, figs. 9a-b. 



Like 0. tricenaria, but much wider in proportion to the 

 length ; hinge-line also longer than greatest width. 



Horizon and localities. Lower Silurian, Girardeau lime- 

 stone : Cape Girardeau. 



Orthis subquadrata HALL. 



Orthis subguadr.ata Hall, 1847 : Pal. New York, vol. I, p. 126, pi. xxxiiA, 



figs. 3a-o. 



Orthis subquadrata Hall, 1862: Geol. Sur. Wisconsin, Ann. Kept., p. 54. 

 Orthis subquadrata Meek, 1873: Geol. Sur. Ohio, Pal., vol. I, p. 94, pi. ix, 



figs. 2b-g. 



Shell attaining about a medium size, rather distinctly 

 resupinate, somewhat wider than long, subquadrate in general 

 outline ; moderately convex ; cardinal margin shorter than the 

 breadth of the valves, and rounding abruptly at the extremi- 

 ties into the lateral margins, which round and converge for- 

 ward ; front a little sinuous or straightened at the middle. 



Dorsal valve more convex than the other, its most promi- 

 nent part being near the middle ; mesial sinus small and rather 

 shallow, sometimes continued back nearly to the umbo, or in 

 other instances scarcely more than reaching the middle ; beak 

 very short, or little distinct from the edge of the area, and 

 more or less arched ; area narrow, directed obliquely backward 

 and downward. Interior with scars of the adductor muscles 

 moderately distinct, the posterior pair being situated close 



