476 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.51. 



When creeping, the foot is fully extended from the anterior end of 

 the shell. It is a narrow, white, ribbon-Hke organ, almost three 

 times as long as the valves. The plantar surface of the foot is on 

 the extreme tip. The movement of the foot is uniform, but the shell 

 is brought up in a series of jerks, as in the Pupillidae, progression 

 being as rapid as in any snail of their size. This movement is easily 

 observed by placing the moUusks in a bottle of water, when they will 

 quickly climb up the sides of the glass. 



Specimens collected early in September had from one to five young 

 in the mnbonal region of the sheU. Each of these had a pair of well- 

 developed valves and appeared very large to have escaped from so 

 small an animal. Before the end of September aU the young seem to 

 have escaped from the parent and lead a free existence during the 

 first winter. 



No definite data can be submitted to show how this and other 

 fresh-water species of animals have come to these islands, which, 

 geologically, are very young, and certainly never have been con- 

 nected with main land. The young of Pisidium does not seem to 

 be adapted for clinging to the feet or feathers of waterfowl, yet this 

 seems to be the only way in which the animals could have been trans- 

 ported over the 200 miles of intervening salt water. 



The new species is much more common than P. scutellatum Sterki 

 which lives in the same ponds. Neither species seems to have been 

 previously collected on the PribUof group. Henry W. EUiott lists 

 Cyclas as an abundant animal on all the islands. Along with it he 

 gives Planorbis, Melania, and Limnea, which are not found there, 

 so that it is questionable whether he really saw any of them.^ 

 Dr. W. H. DaU says: " * * * and probably Pisidium exists in 

 the pools of St. Paul as it does on many of the Aleutians." ^ 

 (G. Dallas Hanna.) 



DESCRIPTION OF SPECIES. 



PISIDIUM HANNAI, new species. 



Mussel moderately inequipartite, slightly to barely obhque, 

 medium inflated, the diameter greatest above the middle; outlines of 

 the edges of the valves broadly eUiptical, the anterior end almost as 

 broadly rounded as the posterior; beaks slightly behind the middle, 

 rather large, somewhat raammiUar, projecting over the upper margin, 

 shghtly flattened on top, outward; surface dulUsh to sHghtly glossy, 

 with slight, shallow, irregular concentric striae, and a few lines of 

 growth; color yellowish to grayish corneous; shell thin, opaque to 

 subpeUucid; hinge rather stout, plate broad, broadest in the middle 

 and with weU-marked sinuses towards the anterior and posterior 



I Seal and Salmon Fisheries and General Resources of Alaska, vol. 3, pp. 19, 227. 

 « Report of Fur Seal Investigations, 1896-97. part 3, p. 541. 



