LOWER HELDERBERG ROCKS. 
271 
Fig. 1. The interior of the right valve of this species, showing the anterior and posterior teeth, the 
ligainental area and muscular impressions. 
Fig. 2. The left valve of an older specimen, in which the anterior teeth appear to have been partially 
obliterated by age. The hgamental area is proportionally wider than in fig. 1. 
Fig. 3. A cardinal view of the exterior of the same species, showing the ligamental area.. 
Palaeaica saffordi. 
Fig. 4. The interior of the right valve, showing the hinge-teeth and ligamental area, muscular im¬ 
pressions, etc. The ligamental area is very narrow in the specimen. 
Fig. 5. The left valve, showing a wider ligamental area, with the anterior teeth less strongly defined 
than in the preceding figure, which represents the prevailing character of this part of the 
shell. The posterior teeth are more oblique and more strongly defined than in fig. 4. 
The posterior muscular impression is but faintly defined, though distinctly visible in 
several specimens, occupying a larger area than the posterior impression; the shell at that 
point being much thinner, and often worn through from the exterior in the specimens 
examined. 
This species, like the preceding, occurs in strata of the age of the Trenton lime¬ 
stone in Tennessee, and approaches in form some of the species in New-York, the 
hinge structure of which is yet unknown. 
There are, also, besides these, some species in the strata under con¬ 
sideration which resemble the Paltearcm of the Lower Silurian rocks, 
both in their general external features and in the large muscular scar. 
In several forms, however, they approach Avicula, and do not appear to 
