LOWER HELDERBERG ROCKS. 



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Paljearca ventrlcosa. 



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 heen partially 



obliterated by age. The ligamcntal area is proportionally wider than in fig. 1. 

 Fig. 3. A cardinal view of the exterior of the same species, showing the ligamental area. 



Palxarca saflbrdl. 



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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 figiire, 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 mu.scular 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 ezterior in the specimens 

 examine<l. 



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 Palajarcae 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 



