PLATE XXX. 
Mytilarca (Plethomytilus) arenosa. 
Page 253. 
See Plate 87. 
Fig. 1. A right valve of medium size. 
Schoharie grit. Schoharie, N. Y. 
Mytilarca (Plethomytilus) ponderosa. 
Page 254. 
Fig. 2. A right valve of a small specimen. 
Fig. 3. A left valve of a larger specimen, showing considerable extension of hinge-line. 
Fig. 4. A left valve of medium size. 
Upper Helderberg group. Clarence Hollow, Erie Co., N. Y. 
Fig-. 5. The under side of the rostral portion of a specimen showing the great breadth of the striated liga- 
mental area. The margin of the shell is compressed and bent inwards "on the anterior side 
showing the surface strife. 
Upper Helderberg group. Columbus, Ohio. 
Figs. 6, 7. The profile and left views of a large specimen entirely denuded of the test. 
Upper Helderberg group. Columbus, Ohio. 
