No. 186.] 



11 



Pal^earca saffordi. 



Fig. 4. The interior of the right valve, showing the hinge-teeth and ligamental 

 area, muscular impressions, 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 jej) resents 

 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 fiihitly defined, though 

 distinctlj" 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 occurs in Tennessee, and, like the preceding, in strata of the 

 age of the Trenton limestone, and approaches in form some of the species in 

 New-York, the hinge structure of which is yet unknown. 



Under this genus may be arranged the following si^ecies from Vol. I, 

 Palaeontology of New- York : 



Amhonychia ohtvsa = Palsearca obtusa. 



Cardiomorpha vetusta 



JCdnwndia suhtrun ata 



E. subangulata 



E. ventricosa 



Modiolopsis latus 



M. subspatulatus 



There are, also, besides these, some species in the Lower Helder- 

 berg rocks, which resemble the Palsearcae 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 have had an external ligamental area. A single cast has, upon the 

 anterior portion of the hinge-line, as many as six or seven crenulations ; 

 while the posterior portion of the cardinal line, in another specimen, 

 shows two long narrow teeth. This structure of the hinge-line, though 



P. 



vetusta. 



p. 



subtruncata. 



p. 



subangulata. 



p. 



ventricosa. 



p. 



lata. 



p. 



subspatulata 



