ART. 20 A NEW ALASKAN GENUS OF MOLLUSKS KIRK 5 



well as the relative position of the anterior adductor is, of course, 

 determined by the very different degree of torsion in the shells. 

 In Pycnodesina there appears to be no pedal scar, which is present 

 in both Megalodon and Megalonius. In Pycnodesma the beaks are 

 much smaller and less conspicuous, the hinge line shorter and the 

 anterior margin longer, all of which give the genus a general aspect 

 quite at variance with Megalodon and Megalomus. 



As genotype, Pycnodesma giganteum^ new species, has been chosen. 



The genus as yet is known only from the high upper Silurian 

 strata of Alaska but doubtless will be found in Asiatic faunas of 

 equivalent age. 



PYCNODESMA GIGANTEUM, new species 



This species is found in great abundance in the massive Silurian 

 limestone of Glacier Bay, notably on Willoughby and Drake Islands. 

 It is also probably the species found in the Silurian limestone of 

 Seward Peninsula and in the massive limestone at the base of the 

 Silurian section in Freshwater Bay, Chichagof Island. 



Extremely large specimens of the species are often seen. On 

 Drake Island, Glacier Bay, I saw a weathered cross section of an 

 individual measuring more than 12 inches (30 centimeters). Judg- 

 ing b}' the thickness of the shell it is probable that this section 

 did not lie along the line of greatest length but gave the approxi- 

 mate height of the shell. The specimens here figured are of small 

 to medium size, as it was not possible to collect the largest specimens 

 in other than a fragmentary condition. 



The shell is obliquely ovate, with the greatest height in the pos- 

 terior portion. The longest dimension of the shell is an oblique line 

 from the beaks to the posterior-ventral margin. In a small indi- 

 vidual, reasonably well preserved, the following measurements ob- 

 tain : Straight portion of hinge, 2 centimeters ; total length of hinge, 

 about 3 centimeters; anterior margin, 4.5 centimeters; height, 6 + 

 centimeters; and greatest length, 7+ centimeters. The greatest trans- 

 verse dimension of this individual is 4 centimeters. These relative 

 proportions seem approximately to hold in the larger specimens. 



The hinge line is short and straight except in its posterior portion 

 where it flexes abruptly downward, meeting and merging with the 

 smoothly and evenly curved posterior margin. The anterior margin 

 is straight and at approximately right angles to the hinge line, or 

 forms an acute angle with it. The ventral margin is relatively short 

 and smoothly curved. The valves are highly ventricose. The umbonal 

 region is prominent and highly arched. The beaks proper are small 

 and sharply incurved. The valve is most highly arched along a line 

 running obliquely from the umbones to the posterior-ventral margin. 

 From this line the shell pitches rather abruptly to the anterior and 



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