ART. 20 A NEW ALASKAN GENUS OF MOLLUSKS KIRK 7 



is characteristic of the typical specimens of the species. The pos- 

 terior muscle scar is very poorly known, as the posterior portion 

 of the shell is usually broken away, and only indifferent internal 

 molds of the shell are available for study. The posterior muscle 

 appears to have been large and situated high up on the shell, below 

 the posterior end of the hinge. 



The type specimens were collected in the massive upper Silurian 

 limestones of Drake and Willoughby Islands, Glacier Bay, south- 

 eastern Alaska. Collector, E. Kirk. 



C oty pes.— C2it. No. 71275, U.S.N.M. 



PYCNODESMA BENJAMINI, new species 



This species is common in the upper portion of the upper Silurian 

 section as exposed in Freshwater Bay, Chicagof Island, south- 

 eastern Alaska. Occurring as it does in argillaceous limestone which 

 is much checked and fractured it is, however, difficult to secure 

 reasonably perfect specimens. Enough material was seen and col- 

 lected, however, to prove that the form is specifically distinct from 

 Pycnodesma giganteum which is found 1,000 feet or more strati- 

 graphically lower in the section. 



The species, though of large size, apparently does not reach the 

 great dimensions of Pycnodesma giganteum. Unfortunately no in- 

 dividual has all the margins sufficiently well preserved to get the 

 relative proportions of the shell and thereby estimate the maximum 

 dimensions of large fragmentary specimens. 



In general outline the shell appears to be subquadrate, with the 

 height somewhat in excess of the length, and with the greatest di- 

 mension the line running from the beak to the posterior-ventral 

 border. The general slope of the shell is very different from Pycno- 

 desvia giganteum. The hinge is proportionally longer, as is the 

 anterior margin. The anterior margin of P. giganteum is at ap- 

 proximately right angles to the hinge line or forms an acute angle 

 with it. In P. henjamini the anterior margin carries forward, mak- 

 ing a decided obtuse angle with the hinge line. 



The hinge line as noted above is proportionally longer than in 

 P. giganteum. In its posterior portion it curves gently downward 

 instead of being sharply flexed. The anterior margin is straight 

 in its upper portion, but ventrad it flares outward. The valves are 

 much less ventricose than in P. giganteum. The umbonal region 

 is the most highly arched portion of the shell. The beaks are small 

 and incurved. The valves are most highly arched along a line run- 

 ning from the beaks to the posterior-ventral margin, but this arched 

 area is not sharply differentiated from the remainder of the valve. 

 Toward the anterior margin the shell pitches off rather abruptly, 

 particularly in the dorsal part. In all other directions the shell 



