2 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 71 



the limestones carrying the Broohsina fauna and is overlain by 

 Middle Devonian. Within the series itself is found a fairly large 

 and varied fauna, which ranges with considerable uniformity from 

 top to bottom. Collections of fossils from the lower portion were 

 made in Glacier Bay and in Freshwater Bay. The fauna of the 

 upper half is chiefly known from the exposures in Freshwater Bay 

 and along the shore between False Bay and lyoukeen Cove. Among 

 several unusual moUuscan types the new pelecypod genus Pycno- 

 desma is of particular interest and stratigraphic value. 



PYCNODESMA, new genus 



This huge pelecypod has a wide range in the Pacific coastal regions 

 of Alaska. It has been found in Seward Peninsula and at various 

 points in southeastern Alaska, including Glacier Bay, Freshwater 

 Bay, Chichagof Island, and the south shore of Kosciusko Island. 

 A pelecypod doubtfully referred to the genus has been collected on 

 Quail Creek in the Rampart region, Alaska. An excellent series of 

 specimens of this Alaskan pelecypod is now available for study. 

 These specimens come in the main from Glacier Bay on the main- 

 land and Freshwater Bay, Chichagof Island, both in southeastern 

 Alaska. It is of interest to note that during the field season of 1926 

 sections of a large thick-shelled pelecypod were found in the high 

 Middle Devonian near Gold Hill, Utah. The horizon is above one 

 carrying String ocephalus. The largest specimen seen had a height 

 of approximately 10 centimeters. The thickness of the shell at the 

 umbones and the structures indicating the presence of a large hinge 

 ligament comparable to that of Pycnodesma suggest the possibility 

 that the genus ranges up into the Devonian. 



In the past this pelecypod has at various times been identified as 

 Megdlomus. Preparations of the Alaskan material and of typical 

 Megdlonrms from the Guelph of Canada show that the Alaskan 

 pelecypod is not referable to Megalomus but constitutes a new genus. 

 Two species are here described, one coming from the massive lime- 

 stones of Glacier Bay and the other from Freshwater Bay, Chichagof 

 Island, in argillaceous limestone 1,000 feet or more stratigraphically 

 above the Glacier Bay horizon. The pelecypod is an excellent hori- 

 zon marker, as owing to its tremendously thick shell it can often 

 be recognized in sections on weathered rock surfaces where meta- 

 morphism and shearing has destroyed other organic remains beyond 

 the possibility of accurate determination. 



Pycnodesma is notable in being perhaps the largest and most mas- 

 sive Paleozoic pelecypod. The individuals here figured are small to 

 medium in size, as owing to the exigencies of collecting in Alaskan 

 localities it was impossible to extract the larger specimens from the 



