8 PEOCEEDINGS OP THE NATIONAL MUSEUM vol.71 



slopes off gradually. The anterior portion of the shell is slightly 

 flattened in its upper part but not so decidely as in P. giganteuin, 

 nor is there the incurving of the valves as in the latter species. 



The shell itself is relatively thinner than in P. giganteum. As in 

 that species the thickest portion of the shell is at the umbones, and 

 continuing backward and downward along the line of greatest con- 

 vexity. The surface of the shell is marked by fine concentric growth 

 lines. The lunule is small and the escutcheon narrow and relatively 

 inconspicuous. 



The internal shell structures are inadequately known owing to the 

 poor preservation of the material. The hinge plate is proportion- 

 ally longer than in P. giganteuTti and straighter. The anterior teeth 

 seem to be similar in all respects to those of P. giganteum^ as is the 

 anterior muscle scar. The posterior teeth and muscle scar are 

 unknown. 



The species may readily be distinguished from P. giganteum. It 

 has a thinner shell. The beaks are more prominent owing to the 

 fact that the umbones are less highly arched. The valves are less 

 highly arched than in P. giganteum, giving a much narrower shell. 

 The most striking difference is the general outline. The obtuse 

 angle formed by the anterior margin with the hinge line and out- 

 ward flare of the lower portion of the anterior margin is a very 

 striking feature. The relatively longer and straighter hinge line is 

 also characteristic. 



The specific name is given in honor of Dr. Marcus Benjamin, of 

 the United States National Museum. 



PycnodesTna henjamini was collected in the upper portion of the 

 upper Silurian section on the south side of Freshwater Bay, Chi- 

 chagof Island, southeastern Alaska, near the head of the bay. 

 Collector, E. Kirk. 



Holotype.— Cat. No. T1276, U.S.N.M. 



EXPLANATION OF PLATES 



Plate 1 



Pycnodesma giganteum, new species 



Fig. 1. Anterior view. 



2. Dorsal view of same specimen. 



3. Exterior, left valve of same specimen. 



4. Interior, left valve of same specimen. 



5. View of right side of another specimen showing almost complete 



outline. Cat. No. 71275, U.S.N.M. 



Pycnodesma giganteum, new species? 



6. Portion of right valve of a specimen showing growth lines. Oat. No. 



71277, U.S.N.M. 

 All figures approximately X 2/3. 



