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Class Gastropoda

Shell permanently cemented to a hard substratum (usually rock), coiled loosely or irregularly and generally twisted, resembling the calcareous tube of a serpulid polychaete more then a snail shell - Family Vermetidae
Shell not permanently cemented to a hard substratum, either tightly coiled or tubular, conical, or cap-shaped - 2

Shell tubular, slightly curved, the height more then twice the diameter - Family Caecidae
Shell conical or cap-shaped, the height rarely exceeding the greatest diameter - 4

Shell with a dorsal opening at or near the apex, or with a slight indentation at the anterior margin (such an indentation is present only in the rare and subtidal Arginula bella; it marks the place where a groove on the interior of the shell, beginning at the apex, reaches the margin) - Family Fissurellidae
Shell with neither a dorsal opening at or near the apex nor an indentation on the anterior margin - 5

Apex of the shell usually at or anterior to the middle, sometimes slightly posterior to the middle (if only the shell is available, the open end of the horsehoe-shaped muscle scar faces anteriorly, except in the family Siphonariidae, in which the open end of a nearly C-shaped muscle scar faces the right side [left side, if the shell is observed in ventral view]) - 7
Apex of the shell decidedly posterior to the middle (at least as far as the beginning of the last quarter) (the foot secretes a calcareous base, and the animal remains attached to this) - Family Hipponicidae: Hipponix cranioides

Outline of the shell, in dorsal view, not symmetrical, the apex slightly to the right of the midline; muscle scar (often indistinct) nearly C-shaped, its open end facing the right side (left side if the shell is observed in ventral view); with a shallow siphonal groove passing through the open end of the muscle scar - Subclass Pulmonata, Family Siphonariidae
Outline of the shell, in dorsal view, symmetrical (unless deformed by an injury), the apex on the midline; muscle scar horsehoe-shaped, its open end facing anteriorly; without a siphonal groove - 8

Interior of shell uniformly whitish, without any color pattern; apex in the anterior third of the shell; exterior with rather conspicuous concentic lamellae (subtidal) - Family Lepetidae
Interior of shell usually with a color pattern of some sort (a blotch in the apical region, marginal markings, etc.); apex in the anterior or middle third of the shell (in the middle third in Acmaea mitra, the only species in which the interior is uniformly white); concentric lines not often conspicuous - Order Patellogastropoda, Families Acmaeidae, Nacellidae, and Lottiidae

Abalones--shell low and earlike, with a low spire near the posterior end, and with a series of holes (some closed) near the left side; length sometimes exceeding 10 cm - Family Haliotidae
Shell generally not low and earlike (except in Lamellariidae and Velutinidae), and without a series of holes near the left side; only a few species larger then 10 cm, and nearly all of these have tall spires - 10

Shell (completely or almost completely internal in Lamellariidae) with a low profile, thus resembling the shell of a abalone - 11
Shell not resembling that of an abalone - 12

Shell thin, translucent white, and completely or almost completely internal - Family Marseniidae
Shell rather firmly calcified, covered by an almost velvety periostracum, and to a large extent external - Family Velutinidae

Outer lip of the aperture with a deep slit ( the shell, which has a short spire above the proportionately large body whorl, resembles that of a Margarites or Lirularia [Trochidae], its diameter being about equal to its height; found only at depths greater then 400 m) Family Scissurellidae
Outer lip of the aperture without a deep slit (there may, hovever, be a notch or distinct siphonal canal at the anterior end of the aperture, and there may also be a slight notch on the outer lip near the posterior end of the aperture) (includes most intertidal and subtidal prosobranch gastropods, and some opisthobranchs and pulmonates) - 13

Length of the aperture nearly equal to the height of the shell (if there is a spire, it is not raised) - 14
Length of the aperture not nearly equal to the height of the shell (there is a raised spire) - 15

Shell thick, similar in shape to that of a cowrie, with a series of teeth on the inside of the outer lip of the aperture; aperture not decidedly wider in its anterior half than elsewhere; height about 3 mm - Marginellidae: Granulina margaritula
Shell thin, not resembling that of a cowrie; without teeth on the inside of the outer lip of the aperture; aperture usually widest in its anterior half; height up to nearly 2 cm, but much smaller in some species - Subclass Opisthobranchia, Order Cephalaspidea

With a prominent siphonal canal that is about 1/3 the total length of the aperture (photo); height up to about 12 cm (or more) - Cymatiidae: Fusitron oregonensis
Without a distinct siphonal canal, but the anterior of the aperture is angled in such a way that it forms a small spout; height generally less than 4 cm - Capulidae (formerly Trichotropidae)

Anterior end of the aperture without a distinct siphonal notch, spout, or canal (photo) 18
Anterior end of the aperture with a distinct siphonal notch, spout, or canal 34

Interior of shell pearly (except in Halistylus pupoideus, family Trochidae); with an operculum (photo) 19
Interior of shell not pearly (it may, however, be colored); with or without an operculum 20

Operculum thin and horny, with numerous spiral lines (photo) Family Trochidae
Operculum calcified and rather thick, with only a few spiral lines Family Turbinidae