49 PALAEONTOLOGY OF NEW- YORK. 



about '18 of an inch. The cast of the ventral valve shows a large flabelliform 

 muscular impression, from the base of which to the apex of the cast is about 

 two-thirds the entire length, while its width is about throe-fifths the width of the 

 shell. The cicatrix for the cardinal muscle is very strong ; and below it is a nar- 

 row sinus, indicating the existence of a strong median ridge. The ventral area is 

 unknown, but it has probably been considerably larger than that of the dorsal 

 valve. 



The description is drawn from the casts of two valves, which, in some parts, 

 preserve a little of the shell. 



The long hinge-line is a characteristic feature of the shell ; while the large 

 area of the dorsal valve, and its well-marked muscular impression, distinguish it 

 from nearly all the other species. The muscular impression of the ventral valve 

 is remarkably large and rigidly flabelliform, with the margins strongly defined. 



The casts are associated in a thin band of limestone of peculiar character, 

 leaving no reasonable doubt of their being of the same species ; while another 

 ventral valve, referred Avith some doubt to this species, was likewise found in the 

 same horizon, in the lower beds of the series. 



This species, in its large muscular impression, is similar to Orthis musculosa of 

 the Oriskany sandstone ; but the dorsal valve is not so convex, the hinge-line is 

 much longer and the dorsal area much larger. 



Geological formation and locality. In the Onondaga limestone : near Williams- 

 ville and Clarence, Erie county, N.Y. 



Ortliis Idas. 



PLATE V. 

 OrthU equivalvu : Haw., Tenth Report of the Regents on the State Cabinet, 1857, p. 109. 



Shell subcircular or subquadrate, the hinge-line equalling two-thirds 

 the greatest width ; sides somewhat straight ; front broadly and re- 

 • gularly rounded, the greatest width a little below the middle. Dorsal 

 valve moderately convex, the greatest convexity above the middle, 

 with a shallow sinus from the beak for half the length of the shell, and 

 curving very gently to the front and lower lateral margins, sloping a 

 little more abruptly in the upper part, and the surface becoming con- 

 cave towards the cardinal angles : dorsal area on a plane with the 

 margins of the shell ; cardinal process large and strong. Ventral valve 



