852 Notes on a Nevada Shell ( Pyrgula nevadensis). [September, 
tion, and most of whom have never seen even the shell. Nearly 
or quite all who have based their opinions upon actual study of — 
shells, have been obliged to content themselves with such exter- 
nal characters as the shells furnish, and these are often most un- 
Satisfactory. Pyrgula is operculated, carinated, and with the spire 
well elevated, thus presenting features which might excuse a 
reference of it to the Melaniidz, which has been actually made by 
most authors. Until the work of Stimpson’ on the Rissoide the — 
genus appears to have been placed in some most remarkable — 
positions. At that time nothing was known except the obvious — 
characters furnished by the test, and some observations by Mo- f 
quin-Tandon on the form common to the rivers of the mountain- 
cus portions of France, —P. bicarinata Bourg. a 
In the system here provisionally adopted from Stimpson, — 
Pyrgula lies between Paludestrina and Tryonia, having super 
ficial characters intermediate to those genera. Dr. W. H. Datt, 
has suggested, in a recent letter, that possibly the Pyramid lake 
shell would be found to rank in Amnicola, basing his ua 
upon the operculum, which certainly possesses some Am ns 
characters, and upon the produced anterior part of the aperture, 
as mentioned in the generic description by Christoforo and Ja. 
But now the work of Mr. Beecher for the first time enables i 
correctly place it among the Rissoide. My. or 
made no comparisons with figures resulting from the Tu 
others, and I hence introduce here a few notes based parr 
observations of Troschel, Moquin-Tandon, Stimpson and my" 
The formula for the rhachidian tooth of the genus Amnic? 
is <4. The tooth itself is about three times broader parr 
Sacer hes oe bee a a E ae MRS cas E E eae Reed Met IN Cac ot 
SRS SERRA Sanh a ea pe Pia ll fle aE ERT EST STO a pS alt en ee ee 
= o #364 uliar ton 
being +. The pecul Pam: 
process of this tooth e ji 
i ing in Pyrgula, an w 
Rhachidian tooth of Ampi. C CY wanting ! fi r ilate, Plie 
cola porata Say. Enlarged lobes are more Gen arge e | 
reee trina has a similar formula OF FT 
chidian tooth—*1-4—but differs in other important pat 
; that genus 
of Pyrgula; the formula mer i 
cola is e 
* Researches on the Hydrobjinæ and allied forms.” Smi SE a 
No. 201, 1865. a : 
3 Loc. cit., p. 47. 
