ORTHIDES OF THE UPPER HELDERBERG ROCKS. 
3 ® 
Surface marked by fine subangular strise, which increase by bifurcation, 
and are strongly curved on the upper margins : spaces between the 
striae wider than the ridges, and, when exfoliated, showing finer striae. 
Six to eight striae in the distance of two lines. 
This fossil presents so many characteristics of 0. livia ( loc. eit.), that I have 
referred it to that species; though in some respects it exhibits differences, which 
may indicate a doubt of its identity. The dorsal valve, which appears to retain 
its normal form, has its greatest convexity above the middle, and the ventral 
valve is more depressed towards the front; but these are characters which I 
conceive may be subject to variation, without indicating specific differences. In 
the specimen described, the ventral valve has been accidentally compressed. 
Compared with 0. vanuxemi , the dorsal valve is not so convex; nor does it 
show a mesial sinus so common in that species, while the beak is shorter, not 
rising so nearly to the same height as that of the ventral valve ; a character 
common in good specimens of that shell. It differs from 0. leucosia in the same 
respect, and in having the area of the dorsal valve flat; while in that species it 
is slightly incurved, and the form is narrow towards the beak. Orthis penelope is 
more oblate, the beaks of the two valves more nearly equal, and the striae in 
this, and the two other species are finer and more rounded. 
The 0. livia has been very elaborately described iff the Canadian Journal, 1. c. 
Geological formation and locality. In the Corniferous limestone of Western 
New-York. 
Orthis sp.? 
PLATE Y. 
Shell orbicular or transversely subelliptical, the length and width about 
as six to seven : cardinal line less than half the greatest width of the 
shell; extremities rounded and curving outwards, the lateral margins 
abruptly rounded, and the basal margin broadly rounded. Dorsal valve 
moderately convex, somewhat flattened in the middle, the greatest 
convexity a little above the middle, curving regularly to the lateral 
and basal margins ; dorsal area very narrow. Yentral valve less convex 
than the opposite, the greatest convexity near the beak, sloping gently 
towards the front and sides, becoming slightly concave towards the 
margins. 
