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LYMN/EIDZ OF NORTH AMERICA. 93 
are its small size, slender form, and costate surface. With the excep- 
tion of the last-mentioned character, it is evidently related to Limnea 
subulata of Sowerby, which, however, is described as having the sur- 
face smooth. 
“Tt is possible that a distinct genus should be established for the 
reception of this little shell, though I have concluded to range it pro- 
visionally as the type of a subgenus under Limnea” (Meek). 
As remarked by Meek, this species is not likely to be confounded 
with any other form of Lymnea, either recent or extinct. The regular 
cost are peculiar, as are also the long and narrow upper whorls. It 
resembles the recent genus Acella Haldeman; the presence of the regu- 
lar coste provide a character of generic importance, as does also its 
peculiar shape. Meek placed it in a separate group, which he called 
Pleurolimnea. 
The species was first called tenwicosta, which name was changed 
in 1860 to tenuicostata, for no apparent reason except, perhaps, that 
of euphony, which is not recognized in specific nomenclature. The 
writer has, therefore, restored the first spelling. 
It is interesting to note that this species extends from the Cre- 
taceous into the Lower Eocene. The Wasatch shell was distinguished 
as micronema by White, but recently Mr. T. W. Stanton has expressed 
the opinion that it is the same as Meek’s tenuicosta. A comparison 
of White’s outline figure (pl. XVII, fig. 4) with Meek’s figure of 
tenuicosta easily leads the student to the same conclusion. This species 
is also noteworthy for its wide geographical range, the most extensive 
of any known fossil species. 
Pleurolimnza tenuicosta whiteavesi. Noy. Var. 
Limnea tenuicostata WuiteaVves, Rep. Geol. & Nat. Hist. Surv. Can., 1886, 
a. Ser. ai; ;p. 163E, 1897. 
Dr. Whiteaves has recorded a variety of tenuicosta from the 
Canadian Laramie, which should bear a name, as it is sufficiently 
distinct from the typical form of the species. The new variety has 
from 18 to 20 fine ribs or coste on the outer whorl, instead of 8 to 
12 as in typical tenwicosta. As this discrepancy would scarcely exist 
in specimens of the same species, it is proposed to give the name 
whiteavesi to the variety in honor of Dr. Whiteaves, the eminent 
Canadian paleontologist, new deceased. 
A careful comparison should be made between this variety and 
the specimens from the vicinity of Township 39 referred to tenuicosta, 
to see if they may not all be of the multicostate form and so referable 
to the variety rather than to the typical form. 
