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 Limncea 

 subulata of Sowerby, which, however, is described as having the sur- 

 face smooth. 



"It 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 Limnaa" (Meek). 



As remarked by Meek, this species is not likely to be confounded 

 with any other form of Lymncea, either recent or extinct. The regular 

 costse are peculiar, as are also the long and narrow upper whorls. It 

 resembles the recent genus Acella Haldeman; the presence of the regu- 

 lar costse provide a character of generic importance, as does also its 

 peculiar shape. Meek placed it in a separate group, which he called 

 Pleurolimncea. 



The species was first called tenuicosta, 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 microncma 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 (pi. 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. 



Pleurolimnaea tenuicosta whiteavesi. Nov. Var. 



Limn&a tenuicostata WHITEAVES, Rep. Geol. & Nat. Hist. Surv. Can., 1886, 

 n. ser.^ ii, p. 163 E, 1887. 



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 costse on the outer whorl, instead of 8 to 

 12 as in typical tenuicosta. 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. 



