LOWER HELDERBERG ROCKS. 
28 3 
Fig. 7. The left valve of this species. 
Fig. 8. The left valve of a smaller individual. 
Geological position and locality. In the pentamerus limestone of the Lower Hel- 
derberg group : Schoharie county. 
Aiicula obliquata (n. s.). 
Plate LI. Fig. 9 & 10. 
Shell very obliquely subovate, arcuate ; anterior side straight above and 
broadly curving below : anterior wing very small; posterior wing large, 
not reaching as far as the margin of the shell, separated by a well- 
defined sinus from the body of the shell, abruptly rounded on the 
postero-basal margin. 
Surface marked by obscure radiating costae and concentric elevated 
lamellae, the latter of which only appear on the wing. 
Fig. 9. The interior of the left valve, showing obscurely the external costae. [ From im¬ 
perfection in the specimen, the sinus in the external margin, between the wing 
and the body of the shell, is represented as too abrupt.] 
Fig. 10. A cast of the valve, which preserves only the remains of the concentric striae. 
Geological position and locality. In the pentamerus limestone of the Lower Hel- 
derberg group : Schoharie county. 
Avicula aequiradiata (n. s.). 
Plate LI. Fig. 11. 
Shell very depressed-convex, subequilateral, pectiniform ; margin regu¬ 
larly rounded below, and sinuate below the wings : wings subequal. 
Surface marked with fine equal radiating threadlike stride, increased by 
dichotomizing or interstitial addition, and crossed by fine concentric 
striae and a few distant undulations, which are more crowded on the 
umbo, and give a wrinkled appearance to that part of the shell. The 
radiating striae are much finer on the posterior wing, and not visible 
on the anterior. 
Fig. 11. An imperfect specimen of the left valve. 
Geological position and locality. In the shaly limestone of the Lower Ilelderberg 
group : Schoharie county. 
