CONTRIBUTIONS TO PALEONTOLOGY. 35 



" ORTHIS CLEOBIS (n.s.)." 



Shell broadly semielliptical or subquadrate, the cardinal line 

 being straight, and extending fully two-thirds the width of the 

 shell ; cardinal extremities rounded. Dorsal valve of a little 

 more than the medium convexity ; the greatest elevation about 

 the centre, from which it slopes almost regularly to the beak, 

 the front, and the lower lateral margins ; while towards the 

 cardinal angles it is more abrupt, and becomes slightly concave. 

 Ventral valve depressed-concave, flattened at the sides, with a 

 broad shallow sinus which becomes deeper towards the margin. 

 Surface finely striated; the striae much curved upwards at the 

 sides. 



In the cast of the dorsal valve, the distance from the beak to the lower 

 edge of the muscular impression is about one-half the length of the valve : 

 the area is in the plane of the lateral margins, comparatively large, having 

 a width of about 0'18 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 

 three-fifths the width of the shell. The cicatrix for the cardinal muscle is 

 very strong ; and below it is a narrow 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 an- 

 other ventral valve, referred with some doubt to the same, was likewise 

 found in the same horizon, in the lower beds of the series. 



This species, in its large muscular impression, is similar to 0. 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 

 Williamsville and Clarence, Erie county. 



ORTHIS ERYNA. 



Orthis equivalvis .- Tenth Report on the State Cabinet, 1857, p. 109. 



Not Orthis cBquivalvis, PaL N.Y. Vol. i, p. 120. 



See .. .. Davidson, Geological JournaL 



This species was inadvertently described under the name Orthis equi- 

 valvis, in the Tenth Report on the State Cabinet. A species under the same 

 name will be found in Vol. iv, PalaBontology of New-York, illustrated on Plate 5; but 

 the name is preoccnpied by an Upper Silurian Bpecies of Davidson. 



