428 Annals of the Carnegie Museum. 



as new, P. progiiathiim and P. piliila (see infra, Descriptions Nos. 



lO-Il). 



In 1918 and 1919 Mr. Carl C. Engberg, of Lincoln, Nebraska, col- 

 lected some Pisidia on Orcas Island, San Juan County, Washington. 

 Among the several species and forms it may be noted that P. abor- 

 tivum exigiium Sterki was found at several places, and seems to be 

 common and even abundant, but not a single typical P. ahortivnm 

 was collected, while in the Bitter Root Valley, Montana, the latter 

 were predominant, only a few of the form exiguum occurring. The 

 mussels have regular, elliptic outlines, and are strongly inflated, 

 globular, rather variable in size (from 2.2 to 1.6 mm. long when full 

 grown), and with nepionic young. It appears that this smaller form 

 is distributed all along the Pacific Slope, and it may be the really 

 typical form, and the larger, less inflated form, possessing somewhat 

 different outlines {ahortivnm) may be specially developed. 



I have described as new one of the species collected by Mr. Eng- 

 berg, calling it P. orcascnsc (see infra, Description No. 9). 



In the same lot were some other specimens, which may possibly 

 represent undescribed species, but there is not enough material at 

 hand to seem to justify their description as new. It may be proper 

 to state that with this material there was a single minute Miiscidinm, 

 2.6 mm. long, nearly globular, but apparently immature. 



With other material. Dr. Frank C. Baker has sent two specimens, 

 labeled "Alaska." One of them was P. idahoense Roper, much like 

 the large form from Seattle, Washington, 10 mm. long, with six or 

 seven strongly marked rest-lines. Other specimens of so-called ida- 

 hoense from localities in Alaska are markedly different, more like the 

 form indianense, and it is possible that the two forms represent dis- 

 tinct species, as has been suggested by some malacologists. The 

 other specimen sent by ]\Ir. Baker was Spharrium tcnnc Prime, well 

 formed. 6.2 mm. long. 



Other correspondents in the western parts of the country have made 

 collections and sent in specimens, which demand further careful study 



