88 CALIFORNIA ACADEMY OF SCIENCES. 



"by comparison with types,'' says that the latter is not 

 so flat a species, and incHnes to call the variety P. tu- 

 midiis Pfeiffer (1839), described as from tropical Amer- 

 ica. There are differences in his description, however, 

 besides the doubt whether a flatter subalpine shell can be 

 allied to a tumid tropical species, while we have some as 

 near like it farther north. 



P. horniiwdiS described as from the coast, at the south- 

 ern boundary of Alaska, latitude 54" 40', and seems more 

 like a small form of P. corncus than of any North Ameri- 

 can shell. It would not be strange if that Eurasian spe- 

 cies could have been transported across the Pacific by 

 birds, and formed a varietal colony on this side. But its 

 alliance is perhaps as close with P. suhcrcnatns, and since 

 it may prove only a small form of the latter, we prefer to 

 use the prior specific name for our flatter irregular vari- 

 ety. If, as Carpenter suggests, there are specimens con- 

 necting P. subcrenatus with P. Iciitus, they may be sup- 

 posed to be still nearer to his P. tiiniciis, but the former 

 name has a year's priority. We have not, however, seen 

 shells from Oregon with the sharp carina and subtriangu- 

 lar mouth of the latter, which, he also suggests, may con- 

 nect with P. lenttis. It is evident, however, that he only 

 described the young, or a small race, of P. tumeus, for 

 while we find them near San Francisco Bay exactly like 

 his Mazatlan types, we also find much larger ones in 

 warmer localities. 



P. TUMENS Carpenter. In studying large numbers of 

 the west coast species from many localities since 1870, it 

 has become necessary to make another change in the 

 Synopsis of Fresh- Water Shells, given in Vol. IV. The 

 first section of Planorbis there given has "only three 

 whorls visible above," a character common to many spe- 

 cies, including P. corncus, and therefore not at all pe- 



