ABT. 20 A NEW ALASKAN GENUS OF MOLLUSKS — KIRK 6 



massive limestone in which they were embedded. In Glacier Bay I 

 have seen a section of Pycnodesma giganteum in the limestone meas- 

 m-ing more than 12 inches (30 centimeters). This may or may not 

 have been the greatest dimension of the individual. As noted under 

 the description of P. giganteum, it is probable this is the height rather 

 than the length of the individual in question. The shell itself is very 

 massive. In the umbonal region a thickness of 2.5 centimeters (1 

 inch) is common, and shells having a thickness of nearly 5 centi- 

 meters (2 inches) have been seen. The shell becomes thinner toward 

 the posterior and ventral margins but at all times is heavy. The 

 surface is ornamented by low concentric growth lines. The lunule 

 is small and inconspicuous. In P. giganteum there is a large well- 

 marked escutcheon. The escutcheon is narrower and less conspicuous 

 in P, henjamini. 



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

 terior portion. The hinge line is short and straight except in its 

 posterior part, where it flexes abruptly downward. The anterior 

 margin is nearly straight or with a slight anterior flare in the ventral 

 portion as vicAved from the side and has approximately twice the 

 length of the hinge. The ventral and posterior margins are smoothly 

 and evenly curved. The greatest dimension of the shell is from the 

 umbones to the posterior-ventral margin. There is no sharply de- 

 fined umbonal ridge, but there is a smoothly rounded area of greatest 

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

 From this area the shell curves abruptly to the anterior margin, 

 which is flattened or even incurved. To the dorsal margin the shell 

 drops off less abruptly but more so than to the ventral and pos- 

 terior margins. The umbones are highly arched and as noted else- 

 where the shell is greatly thickened in this region. The beaks are 

 relatively small and inconspicuous. They are sharply incurved 

 toward the anterior end of the shell. 



The hinge plate is massive and straight on the dorsal margin ex- 

 cept in the extreme posterior portion, where it flexes abruptly down- 

 Avard. There are four or more massive, long, parallel cardinal teeth 

 in each valve which lie at nearly right angles to the dorsal margin. 

 There is always one well-defined posterior-lateral tooth which takes 

 the form of a long narrow ridge, with a complementary groove on 

 the opposite valve. In addition to this ridge there is a variable 

 development of more or less amorphous interlocking structures of 

 low relief. In the posterior portion of the hinge plate there may be 

 two or more short ridges paralleling the downward flexed portion 

 of the dominant posterior-lateral. These auxiliaiy teeth show par- 

 ticularly well in moderately young individuals. In large specimens, 

 aside from the long lateral tooth, there is a variable development 

 of lumps and ridges of low relief, with the complementary depres- 



