.ORTHIDES OP THE UPPER HELDERBERG ROCKS. 



Surface marked by fine subangular striae, 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 strise. 

 Six to eight striae in the distance of two lines. 



This fossil presents so many characteristics of O. livia ( loc. cit.), that I have 

 referred it to that species ; though in some respectsjt 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 strite in 

 this and the two other species are finer and more rounded. 



The 0. livia has been very elaborately described in the Canadian Journal, 1. c. 



Geological formation and locality. In the Corniferous limestone of Western 

 New-York. 



Oithis sp.? 



PLATE V. 

 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. Ventral valve less convex 

 than the opposite, the greatest convexity near the beak, sloping gently 

 towards the front and sides, becoming slightly concave towards the 



