Monadenia fidelis fidelis (Gray, 1834)

Pacific Sideband


• Helix fidelis Gray 1834: 67. Vancouver, Washington, USA; subsequently selected by Pilsbry (1939).

• Helix Nuttalliana I. Lea 1838: 88, pl. 23, fig. 74 [shell]. “Fort Vancouver down to the Ocean, Oregon”.

• Helix Oregonensis I. Lea 1838: 100, pl.23, fig. 85 [shell]. “Wahlamat, near junction with Columbia River”.

?• Aglaia fidelis var. minor W.G. Binney 1885: 121, 493, f. 91 [shell]. The Dalles, Oregon, USA.

• Monadenia semialba Henderson 1929: 80. Rosario State Park, Fidalgo Island, Washington, USA.

?• Monadenia fidelis celeuthia Berry 1927: 122, f. 2 [shell]. Trail, Rogue River valley, Jackson Co., Oregon, USA.

?Monadenia fidelis columbiana Pilsbry 1939: 43, fig. 16g, h [shell]. Near the Salmon River, 12 mi from Mt Hood, at Tawney’s Hotel, 1600 ft elevation [Oregon, USA].


Monadenia fidelis

Diagnosis and description

Diagnosis: The combination of large, depressed-heliciform form, umbilicus and general colour pattern of banding make this species unique in Canada. Cepaea nemoralis generally has a more brightly coloured and smaller shell that lacks an umbilicus in adults.

Shell: Large; growth determinate; mature w: 22.1–36.5 mm; average size (n=30, mean and standard deviation in parentheses): w: 32.0 mm (4.16), h: 21.1 mm (2.96), h/w: 0.7 (0.03); depressed-heliciform; moderately thick-shelled, opaque. Whorls: 5¼–6¾, moderately convex. Spire: conic, a little variable in relative height; sides ± straight. Apex: rather pointed. Suture: moderately indented. Last whorl: descending close to the mature peristome. Periphery: rounded, medial on last whorl. Protoconch: 1¾ whorls, initially smooth then microscopically granular. Teleoconch: with irregularly sized and spaced, round-topped colabral wrinkles (strongest on the apical surface), and shallow, wavy, microscopic striae (most evident on base). Periostracum: thin, varnish-like on base, matt on apical surface, often with subtle, oblique, zigzag wrinkles. Umbilicus: rather small, ca ⅛ of w, partially occluded by the columellar lip. Aperture: subovate, aw > ah. Peristome: incomplete; viewed from side, prosocline and slightly arched. Aperture dentition: none. Baso-columellar lip and lower portion of palatal lip a little thickened and slightly recurved. Parietal callus: glazed, transparent. Shell (with periostracum): usually chestnut brown or rusty yellowish brown, with a narrow, pale yellow band just below the periphery, a slightly wider, dark brown band above this, and a dark brown base (that is often slightly greenish towards the lip); aperture with the exterior pigmentation clearly showing through a milky whitish enamel; apertural lip pale brown or purplish-brown. Straw-yellow shells, with faint banding, sometimes occur.

Monadenia fidelis    Monadenia fidelis


Monadenia fidelis

Monadenia fidelis


Animal: Body rosy brown, with the spaces between the tubercles greyish; behind the head are some sparse, black reticulations. Mantle with a bold rust-coloured band; sole of foot pale grey.

Anatomy: The reproductive anatomy is figured and described by Pilsbry (1939) and Schileyko (2004).


Range and habitat

Global range (of the species as a whole): Sitka, Alaska (Dall 1905); British Columbia to northwestern California (Roth &  Sadeghian 2003, 2006). La Rocque (1953) listed the species from the Yukon, but the presence of this species there seems unlikely. The Alaska record needs verification.

Canadian range: All around the Georgia Basin; north to Growler Cove, West Cracroft Island, Broughton Archipelago (Dall 1905). There are no records in B.C. north of the Broughton Archipelago. 

Ecozones: Pacific Maritime.

Habitat: In deciduous, mixed or coniferous forests generally, but also sometimes in open woods and grassy places, such as Garry Oak (Quercus garryana) meadows and seashore sand spits. On a small, rocky islet in the Gulf Islands, Monadenia fidelis was found under stunted ‘mats’ of oak (Forsyth 2004; RBCM collection). Lord (1866) reported this species up to elevations of 6,000 ft [ca 1,800 m] although it is predominantly a species of the coastal lowlands. Adult snails are most often encountered in spring when crawling in the open on the ground or climbing up the trunks of shrubs and trees. Jackson (1923) observed M. fidelis as high as 22 ft (6.7 m) above the ground.



This species presumably feeds on fungi and plants. Animals readily ate a varied diet in captivity but diet was not correlated to shell colour (Roth 1980). In captivity, individuals of Monadenia fidelis flava reached adult size in 2.5–2.75 years and survived up to eight years (Walton 1963, 1970, as M. fidelis beryllica). Monadenia fidelis is a host of the parasitic ciliate protozoan Tetrahymena limacis (Warren, 1932) (Kozloff 1956).

In B.C., the smallest shells have been found on open, seaside habitats, such as offshore islands and spits with no or minimal tree and shrub cover. The pale, yellow form is interspersed with normally coloured individuals, but generally quite rare, although at one locality on Pender Island, individuals with yellow shells are rather frequent.



Various subspecies and forms have been described for this variable and relatively widespread species (Pilsbry 1939), with the greatest expression of variation found in the southern part of the species' range (Roth 1980). Some of these infraspecific taxa continue to be used in the recent literature as subspecies (Monadenia fidelis flava (Hemphill in W.G. Binney, 1892), M. f. leonina Berry, 1937, M. f. pronotis Berry, 1931, and M. f. smithiana Berry, 1940; Roth & Sadeghian 2003, 2006), while genetic studies of other subspecies have shown them to be synonyms of M. fidelis or  synonyms or subspecies of M. infumata (Gould, 1855) (see Roth & Sadeghian 2003, 2006). The above synonymy is exclusive of these subspecies and their synonyms.

Monadenia semialba was described from Rosario State Park, Fidalgo Island, Washington for a form in which the dark pigment on the base is much reduced and the pale, generally straw yellow subperipheral band is expanded onto the base. Pilsbry (1939) treated this taxon as a subspecies, as did Eyerdam (1937) before him, but semialba is mixed with typical fidelis at the type locality and the outward appearance of the animals are identical (Eyerdam 1937). For these reasons, Monadenia semialba is regarded as a synonym. Helix nuttalliana, Helix oregonensis, and Monadenia fidelis columbiana are included here as synonyms, the first two names on the authority of Pilsbry (1939). The validity of Aglaia fidelis minor (now M. fidelis minor) , M. f. celeuthia, and M. f. columbiana, have not been recently assessed.

British Columbia populations likely all belong to the nominal subspecies.

Etymology: Latin, “dependable”.



Species bibliography at

Account, map and photo gallery at

BC Species and Ecosystems Explorer


Literature Cited

Berry, S.S. 1927. A new Oregonian subspecies of Monadenia fidelis. The Nautilus 40 (4): 122–124 [29 Apr].

Binney, W.G. 1885. A manual of American land shells. United States National Museum, Bulletin 28: 528 p.

Dall, W.H. 1905. Land and fresh water mollusks. Harriman Alaska Expedition 13: 1–171, pl. 1–2.

Eyerdam, W.J. 1937. Monadenia semialba Henderson. The Nautilus 51 (2): 63–65 [22 Oct].

Forsyth, R.G. 2004. Land Snails of British Columbia, Royal BC Museum Handbook. Victoria: Royal British Columbia Museum. iv + 188 p., [8] pl.

Gray, J.E. 1834. [Mr. Gray exhibited an extensive series of land and fresh-water shells which he regarded as hitherto undescribed]. Proceedings of the Zoological Society of London 2 (1834): 63–68.

Henderson, J. 1929. Some new forms of non-marine Mollusca from Oregon and Washington. The Nautilus 42 (3): 80–82.

Jackson, R.W. 1923. Epiphragmophora a tree climber. The Nautilus 36 (4): 144.

Kozloff, E.N. 1956. Tetrahymena limacis from the terrestrial pulmonate gastropods Monadenia fidelis and Prophysaon andersoni. Journal of Eukaryotic Microbiology 3 (4): 204–208.

La Rocque, A. 1953. Catalogue of the Recent Mollusca of Canada. National Museum of Canada, Bulletin 129: x + 406 p.

Lea, I. 1838. Description of new freshwater and land shells. Transactions of the American Philosophical Society (new series) 6: 1–154, 24 pl.

Lord, J.K. 1866. The naturalist in Vancouver Island and British Columbia. Vol. 2. London: R. Bentley.

Pilsbry, H.A. 1939. Land Mollusca of North America (north of Mexico). Volume 1, Part 1. The Academy of Natural Sciences of Philadelphia, Monographs 3: i–xvii + 1–573 + i–ix.

Roth, B. 1980. Shell color and banding variation in two coastal colonies of Monadenia fidelis (Gray) (Gastropoda: Pulmonata). The Wasmann Journal of Biology 38 (1–2): 39–51.

Roth, B., and P.S. Sadeghian. 2003. Checklist of the land snails and slugs of California. Santa Barbara Museum of Natural History Contributions in Science 3: 81 p.

Roth, B., and P.S. Sadeghian. 2006. Checklist of the land snails and slugs of California. Second edition. Santa Barbara Museum of Natural History Contributions in Science 3: 82 p.

Schileyko, A.A. 2004. Schileyko, A.A. Treatise on Recent terrestrial pulmonate molluscs, Part 12: Bradybaenidae, Monadeniidae, Xanthonychidae, Epiphragmophoridae, Helminthoglyptidae, Elonidae, Humboldtianidae, Sphincterochilidae, Cochlicellidae. Ruthenica Suppl. 2: 1627–1764.

Walton, M.L. 1963. Length of life in west American land snails. The Nautilus 76 (4): 127–131.

Walton, M.L. 1970. Longevity in Ashmunella, Monadenia and Sonorella. The Nautilus 83 (3): 109–112.