Pristiloma, minute land snails of the Pacific Northwest, Identification

Identifying the species of a genus of minute land snails
in the Pacific Northwest
by Barry Roth, Consultant
745 Cole Street
San Francisco, CA 94117
The snail shell
History and taxonomy of Pristiloma
Identifying Pristiloma species
Re-verification of OSAC specimens
to review land snail specimens previously
identified as belonging to the genus
Pristiloma Ancey, 1887, from the collections
of the OSAC and to verify or re-identify them
as needed. Of particular interest is the
putative distinction between Pristiloma
arcticum arcticum and what was known at
the time as P. a. crateris.
1. Introduction
191 samples previously identified as
belonging to the following taxa:
Pristiloma arcticum (Lehnert, 1884) (3 samples);
P. arcticum arcticum (Lehnert, 1884) (19 samples);
P. arcticum crateris Pilsbry, 1946 (98 samples);
P. arcticum n. ssp. (2 samples);
P. idahoense Pilsbry, 1902 (1 sample);
P. lansingi (Bland, 1875) (38 samples);
P. pilsbryi Vanatta, 1899 (1 sample);
P. sp. (20 samples);
P. stearnsii (Bland, 1875) (8 samples);
P. subrupicola (Dall in Packard, 1877) (1 sample).
Specimens examined under stereoscopic microscope at
magnifications of 60X, 120X, and occasionally 240X.
Orientation of the shell with the aperture on the right and
opening toward the viewer:
For stable viewing position, specimens examined in a small
dish of black paraffin in which was impressed a groove 5
mm long and about 0.5 wide. Shell specimens were placed
in this groove and adjusted with an artist’s camel-hair
brush until positioned correctly
Ref: Pilsbry (1946)
Practically everyone has an image in mind of
what a snail looks like…
2. The snail shell
Ref: file photo
species of
In a variety
of shapes
and sizes
Ref: Kerney & Cameron (1979)
systems of
garden snail,
Ref: Kerney & Cameron (1979)
Each rotation around the axis = 1 whorl:
Ref: Kerney & Cameron (1979)
Apical (top) view of snail shell
Ref: mod. after Kerney & Cameron (1979)
Parts of
a snail
Ref: Kerney & Cameron (1979)
Ref: Kerney & Cameron (1979)
Ref: Pilsbry (1946)
◦ A genus of the Northern Hemisphere.
◦ Member of family Pristilomatidae (but sometimes placed in
family Zonitidae).
◦ First species found and named in 1875.
◦ First recognized as a genus by Ancey, 1886.
◦ Approximately 16 species, most diverse in western North
◦ Intensively studied by H. B. Baker (1931), followed by H. A.
Pilsbry (1946).
3. Pristiloma: History & taxonomy
Pristiloma, subdivisions:
◦ H. B. Baker (1931):
◦ Subdivided genus into 3 subgenera and 5 “sections.”
◦ Based on characters of the reproductive system
(accessible only by dissection).
◦ Reproductive system characters not important in
identification of species.
◦ Species can be told apart by features of the shell.
First step:
◦ Does the shell have an umbilicus or not?
◦ PNW species that have an umbilicus:
 P. chersinella
 P. wascoense
◦ Species without an umbilicus:
 P.
 P.
 P.
 P.
 P.
 P.
johnsoni (not present in project samples)
4. Identifying Pristiloma species
The two umbilicate species examined in
project material:
P. chersinella – shell breadth 3-4 mm; range
from southern Oregon (Klamath County) to
southern Sierra Nevada, California.
P. wascoense – shell breadth about 2 mm;
range southern Washington (Klickitat County
and northern Oregon (Wasco County)
Pristiloma chersinella (a) and
P. wascoense (b)
Ref: Pilsbry (1946)
Among project material, two species that lack
an umbilicus are sculptured on the whorls of
the spire with distinct radial grooves:
P. pilsbryi – with deep, rather coarse
grooves, distinctly “bumpy” seen in top view;
range from Pacific County, Washington to
northwestern Oregon (Tillamook County).
P. stearnsii – with shallower, more crowded
grooves; range from southern Alaska to
southern Oregon (Curry County)
Pristiloma pilsbryi
Ref: Pilsbry (1946)
Pristiloma stearnsii
Ref: Pilsbry (1946)
Among project material, three species that lack an
umbilicus lack distinct radial grooves on the whorls of the
P. idahoense – with low spire and deeply dished, bowlshaped base; up to 6.5 whorls, narrow mouth, without an
internal rib. Idaho.
P. lansingi – with low spire and deeply dished, bowl-shaped
base; up to 5.5 whorls, with a toothy rib along the basal
and outer margins of the lip at adulthood. British Columbia
to northwestern California.
P. crateris – with widest part of the shell near middle of
body whorl (“median”) and slope of the shell below the
periphery more rounded, leading to a less deeply “dished”
base. Western slope of Cascade Range from northern to
southern Oregon, questionably in Washington.
Pristiloma idahoense
Ref: Pilsbry (1946)
Pristiloma lansingi
Ref: Pilsbry (1946)
Pristiloma crateris
Ref: Pilsbry (1946)
A major question was the distinction, if any,
between Pristiloma arcticum arcticum and P. a.
crateris. (Following most prior literature,
including Pilsbry’s 1946 monograph, the two
were regarded as subspecies of one species, P.
Of 19 samples previously identified as P.
arcticum arcticum, only one sample (WEN01001) was arguably identifiable as that taxon.
The rest were either P. lansingi, P. crateris, or
tentatively identifiable as juvenile P. stearnsii.
5. Re-verification of Pristiloma
In the most recent monograph of the genus (Pilsbry, 1946),
Pristiloma arcticum crateris was described as follows:
The shell is imperforate, depressed, with quite low, conoid spire
and rounded periphery, median in position; pinkish buff, glossy.
Sculpture of weak but subregular ripples of growth below the
suture, soon disappearing, leaving the peripheral region and base
smooth except for very weak lines of growth; very fine, close
spirals are seen on the upper surface. The whorls are regularly
and rather closely coiled, the last not unduly wider. The aperture
is narrowly crescentic, the outer and basal margins of the lip thin,
columellar margin slightly spreading, thickened within, reflected
at the insertion in a small callus over the axis. Height 1.5,
diameter 2.75 mm; 5 ½ whorls. H./d. index about 54.5.
It is very similar to P. arcticum, but the base is more flattened,
producing a less deeply concave basal lip and somewhat different
shape of the aperture, and there is a fraction of a whorl more
(Pilsbry, 1946: 402-403).
The purported subspecies was contrasted with P. a. arcticum,
which was described as follows:
“Shell imperforate, globose-depressed, most minutely striate,
uniform tawny-brown, glossy. Whorls 5 ½ to 6, convex, very
narrow, the last somewhat convex at base. Aperture depressed,
lunar; peristome simple, acute, the basal margin arcuate. Width
2 mm., alt. 1.5 mm.” (E. Lehnert.)
It is a glossy shell, with the general shape of P. lansingi; growthstriae faint, spire low-conic, whorls 4 ¾, slowly and regularly
increasing, and last not disproportionately wide as in P. johnsoni,
but about as in P. lansingi. Aperture narrowly crescentic. … The
width of the spire a little exceeds two-thirds the greatest
diameter of the shell (Pilsbry, 1946: 401-402).
Pristiloma crateris
Pristiloma arcticum
Distinction between P arcticum
and P. crateris
Ref: Pilsbry (1946)
Most samples previously identified as P.
arcticum crateris (96) were confirmed as
crateris by this study; two others were reidentified as P. lansingi. Pristiloma crateris
(the argument for considering it a species
rather than a subspecies of P. arcticum is
given below) is a readily recognizable species
with a distinctive range within the study
Conclusions, 1: P. crateris is a
readily recognizable species in the
study area.
All specimens herein identified as crateris are
consistent in form and deployed within a
relatively continuous, delimited range. This
is consistent with their representing “a
separately evolving metapopulation lineage,
where a metapopulation is an inclusive
population made up of a set of connected
subpopulations and a lineage is a population
extended through time or an ancestraldescendant series of time-limited
(instantaneous) populations” as in the
definition of a species by de Queiroz (2005).
Conclusions, 2: P. crateris is a
species, not a subspecies
At the present time, we have insufficient information
to demonstrate intergradation between crateris of
this study and arcticum from elsewhere (which would
be suggestive of a subspecific relationship), or
identity of the reproductive systems of undoubted
arcticum from the North Slope of Alaska with those of
specimens from the Pacific Northwest. In the
absence of such additional evidence demonstrating a
subspecific relationship, the proper course is to
recognize two different species: Pristiloma arcticum
and Pristiloma crateris.
Only P. crateris is definitely represented in the
material considered by this study. More material
would be needed to address the question of P.
arcticum occurring in Washington State.
Conclusions, 3
Baker, H. B. 1931. Nearctic vitreine land snails. –Proceedings of
the Academy of Natural Sciences Philadelphia 83: 85-117, pls.
de Queiroz, K. 2005. Different species problems and their
resolution. BioEssays 27: 1263-1269.
Kerney, M. P. & R. A. D. Cameron 1979. A Field Guide to the Land
Snails of Britain and North-West Europe. William Collins Sons:
London. 288 pp.
Pilsbry, H. A. 1946. Land Mollusca of North America (north of
Mexico). Academy of Natural Sciences of Philadelphia,
Monograph 3 2(1): i-viii, 1-520, frontis.

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