ORTHIDES OF THE UPPER HELDERBERG ROCKS. 41 



middle, with tlie apex more produced than in that species. These casts 

 give indications of a median sinus with an elevation on each side of it, 

 and thence an abrupt slope to the cardinal extremities, and curving to 

 the front and lower lateral margins. 



In this character, they more nearly resemble the dorsal valves of Orthis leucosia; 

 ■while the muscular impressions of the ventral valves, from the same locality, 

 are broader than in that species. The hinge-line of a ventral valve is three-fifths 

 the greatest width of the shell, which is greater than in either 0. vantixemi or O. 

 leucosia, while the striae are also much coarser. This specimen occurring with the 

 others, while no other form of ventral valve has been seen in the same association, 

 indicates the probable relations of the other specimens. 



Although the material in my possession is too imperfect for a complete de- 

 scription, I have indicated the prominent features of the species, in order to call 

 attention to the occurrence of this form in the limestones of the Upper Helder- 

 berg group. 



Geological formation and locality. In the Onondaga limestone : near Clarence 

 hollow, Erie county, N.Y. I have seen a similar form from the limestone near 

 Columbus, Ohio. 



Oitliis cleobis ( n. s.). 



PLATE V. 



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 having 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-convex above, flat-* 

 tened at the sides, with a broad shallow sinus which becomes deeper 

 towards the margin. 



ScEFACE 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 

 [ Pal^sontolooy IV.] 6 



