ORTHIDES OF THE UPPER HELDERBERG ROCKS. 
41 
middle, with the 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. vanuxemi or O. 
leucosidy while the strise 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. 
Orthis cleoMs (n. s.). 
PLATE Y. 
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. 
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 
[ Paleontology IV.] • 6 
