CHEMUNG GROUP. 
269 
122 . 
1. Delthyris mesastrialis. 2. Delthyris mesacostalis. 3. Delthyris disjuncta. 
1. Delthyris mesastrialis , n. s. — Semicircular ; hinge line extended into short, acute ears ; 
upper valve broadly emarginate in front; mesial fold very broad and deep, finely striated, and 
well defined; surface marked by from fourteen to twenty rounded ribs on each side the 
mesial fold; ribs prominently rounded and finely striated longitudinally. The ribs are some¬ 
times crossed by a few elevated laminae of growth, as in fig. 1, the upper valve; while in 1 a, 
the lower valve, they are not visible. 
The beautifully striated mesial fold is alone sufficient to distinguish this shell from any other 
in the group. Of two others with a striated mesial fold, one has flat and the other very sharp 
ribs. 
Locality —Cayuta creek. 
2. Delthyris mesacostalis , n. s. — Sub-rhomboidal; beak very much elevated above the 
hinge line ; hinge extremities terminating in abruptly acute ears ; lower valve very convex; 
mesial sinus angular, deep, and with an obtusely angular rib along the centre ; surface marked 
by about fourteen obtusely angular ribs on each side the mesial fold. 
The angular rib in the centre of the mesial fold, with the nearly equal length and breadth 
of the shell, are the distinguishing characters. 
Locality —Angelica, in grey sandstone. 
3. Delthyris disjuncta1 (Phillips, Palaeozoic Fossils, pi. 29, fig. 128,/, g, h, and 129; 
pi. 30, fig. 129.) — Semicircular, convex, broadly emarginate in front; hinge line extended 
into short acute ears ; mesial fold broad, not defined at the borders; surface marked by 
twenty-five to thirty small plain ribs on each side the mesial fold; radii on the mesial fold 
smaller, and bifurcating toward the margin, where there are often fifteen in number. 
