284 PALEONTOLOGY OF NEW-YORK. 
Fig. 3 a. A left valve of ordinary size, where the extremity of the cardinal line is extended 
slightly beyond the width of the shell. 
Fig. 3 b. A larger shell, where the cardinal line is less extended than the width of the shell. 
Fig. 3 c. A fragment of shale with the right valves of two individuals, showing a greater exten¬ 
sion of the cardinal line, and a more distinct wing than in the left valve. 
Position and locality. In the shale at Lockport; often occurring in the same specimen 
with A. emacerata. 
646. 10. AVICULA? ORBICULATA («. sp.). 
Pl. LIX. Fig. 4. 
Compare Avicula orbicularis, Sowerby in Murchison, Sil. System, pag. 635, pl. 20, fig. 2 & 3. 
Suborbicular, depressed, rounded anteriorly ; posterior wing short; surface concentrically 
striated. 
Several fragments of this species have fallen under observation, but they are for the most 
part too obscure for identification. The shell is large and suborbicular in its perfect condition, 
convex in the middle and upper part, with elevated umbones. The specimen figured is com¬ 
pressed, the shell almost entirely removed, and the outlines of the general form and the 
concentric striae only partially preserved. 
Position and locality. In the shale at Rochester. 
647. 2. POSIDONOMYA? RHOMBOIDEA (n. sp.). 
Pi.. LIX. Fig. 5. 
Rhomboidal, inequilateral, alate posteriorly, with an oblique ridge from the beak to the pos¬ 
terior basal margin; cardinal line straight, less than the length of the shell; base rounded • 
anterior side short; surface marked by concentric strise, and on the centre of the shell by 
stronger undulations. 
This shell approaches in many of its characters the preceding species, but the short hinge¬ 
line and erect character of the shell indicate a different genus. It is allied to Posidonomya ? 
alata of the Clinton group, but is an entirely distinct species. 
Position and locality. The only specimen which I have seen is from the collection of the 
Rev. Mr. Walcott, and was found at Lockport. 
648. 22. MODIOLOPSIS? UNDULOSTRIATA ( n. sp.). 
Pl. LIX. Fig. 6 a, 6. 
Subrhomboidal, longer than high ; surface marked by strong subimbricating ridges or folds, 
and intermediately by fine undulating striae. 
