How to use the Terrestrial Mollusc Key


This key was created to assist inspectors at U.S. ports of entry to determine the identity of terrestrial mollusc species intercepted in imported cargo and shipments in-transit. Some terrestrial molluscs are important agricultural and ecological pests, and others may be contaminant species that may materially affect the quality of cargo while others may be non-pest “hitchhiker” species. It is often difficult for non-experts to accurately determine identity of terrestrial molluscs in a timely manner. This interactive key is designed to be a user-friendly tool to aid non-malacalogists to identify some important mollusc species that may affect commerce. It is however acknowledged that the scope of this key may be larger, and as such may also be used as an educational resource in a variety of fields. The taxonomy of terrestrial molluscs is very dynamic; hence, a large number of the entities (species, family, groups) included in this key may have been, and continue to be, revised. For each entity, a list of synonyms has been included in the supporting fact sheets to assist in clarifying the nomenclature.

It is recommended that the user read the ‘Identification’ and the ‘Biology’ sections of this tool in order to use the key more effectively. Only adult specimens have all the characters required by the key to achieve correct identification. Juveniles of many gastropod species often lack adult characters or they may possess additional characters that are not maintained through to adulthood. This is true for both snails and slugs. Slugs are generally more difficult to key to the species level and often require dissection. If dissection is necessary, there is a dissection tutorial available in this tool that will be able to assist the user to successfully dissect a snail and/or a slug.

***** It is important to remember that this key is not inclusive of all pestiferous mollusc species. This key is intended to serve as an aid in the identification of terrestrial mollusc species documented as major agricultural and ecological pests as well as contaminant and non-pest species that are commonly intercepted at U.S. ports of entry. *****

Equipment required for the optimal use of this key:

  • Hand lens (10-20X)
  • Ruler or Caliper
  • Adult specimens
  • Anatomy drawings (located in ‘Biology’ and the ‘Identification’ section)

How to Use

This non-dichotomous key is comprised of 33 families and 129 species. This key provides the user with multiple identification characters that can be selected at any time (in any order), as opposed to having sequentially paired options characteristic of a traditional dichotomous key.

TMT Key Window

S&S Window

The browser window is divided into quadrants.

  • Upper left window: Characters Available – this is a list of the features and states to select for the identification of specimens of interest.
  • Upper right window: Taxa Remaining – list of families, genera and species that could possibly be the specimen of interest.
  • Lower left window: Characters Chosen – list of features or states currently selected.
  • Lower right window: Taxa Discarded – list of families, genera and/or species discarded from the characters available section based on the states previously selected.

Initiating Key

Initating the Key The first character in the key will be a dependency character, meaning that subsequent selections in the key are based on this character. This dependency character state is used to discriminate between a snail and a slug to allow the end user to progress through the key more efficiently.

Subsequent Selections

subsequent selections Each feature may have two or more character states. The selection options can be made visible by clicking on the arrow at the left corner of each character state. The desired character state can be selected by clicking directly onto the thumbnail photograph or illustration or the checkbox located adjacent to the character state. Also note that multiple selections can be made within each feature/state (e.g., in the above diagram filamentous and tubular can both be selected).

There is a feature called “find best”, that can be useful. It is represented by the magic wand icon at the top of the page. This function will select the best characters that are most useful in separating the remaining taxa.

expand selection

Each state’s photograph can be expanded by selecting the icon expand icon located at the bottom right of the photograph. This is indicated in the diagram above.

Final Selection

final selection

The Identification Process

The keying process is complete when there is only a single selection remaining. Selecting additional character states at this point will prove to be futile as there will be no other taxa to discard. If the selection is not satisfactory, the organism can be taken through the key again, this time using other character states. Also the final selection may not be identical (color and markings) to the specimen in hand, as many gastropod species are morphologically variable. It is therefore recommended that the user read the fact sheet on the final selection to confirm the identification as additional information, including pictures, are provided in this section of the tool.

Selecting Characters

One important feature of a Lucid key is the flexibility it provides the user to choose morphological characters pertinent to the specimen in question in any order and in any combination. The user is however advised to select obvious characters first before progressing to more obscure characters. Additionally multiple characters may be selected simultaneously and unclear characters and those difficult to determine may be skipped.

Measurable characters such as height, width, length, number of whorls etc, are very useful to quickly reduce the number of “Entities Remaining”. It is therefore extremely important to correctly measure the specimen in question. The user should consult the “Identification” page for the correct procedures to measure terrestrial gastropods. Do not estimate measurable characters or guess any other character selections. When in doubt choose alternative character states.

Dissections and Comparing Genitalia

Several entities included in this key cannot be reliably identified using external morphological characters additionally some entities may be grouped in a species complex and require examination of genital structures for correct identification. If the specimen in question falls into the aforementioned categories and identification to a lower taxonomic level is desired the user has several options. The user could send the specimen to an expert for official identification. OR Based on skill level and appetite for adventure the user may choose to dissect the specimen in question in order to inspect the genitalia. The user should consult the dissection page for the supplies required and the procedures for correctly and humanely preparing the specimen. The slug and snail dissection tutorial provide a step-by-step guide to the dissection process. The specimen in question may be quite different from those used in the tutorials but the principles are the same. The fact sheets provide detailed illustrations of characteristic genitalic structures required for identification of entities to the species level. The dissected genitalia of the unidentified specimen should be compared to pertinent illustrations in order to make a determination.

Entity not in Key

It is probable that on occasions the end-user will be unable to make an appropriate final selection on the first attempt to key a specimen. The end-user would be advised to attempt to ‘re-key’ the specimen by selecting a different suite of characters or by selecting the “magic wand” feature after the dependency character, within the key. If no selection can be made after a reasonable number of attempts the user must consider the possibility that the specimens on hand may be a species not included in the key. Additionally, the specimen may be a juvenile, the shell may be severely weathered (snails) or the specimen may be an albino morph and therefore lack the characters required to make a final selection.

Multiple Entities Remain

The occasion may arise where the user has exhausted all possible selections from the “Features available” quadrant of the key window yet multiple entities remain in the “Entities Remaining” quadrant. The most likely explanation is that the specimen in question is not an entity included in the key. There may also be occasions where multiple entities remain in the “Entities Remaining” quadrant of the key window and multiple characters remain in the “Characters Remaining” quadrant but none of the remaining features are pertinent to the specimen in question. In such a case it is possible that the specimen in question could be a juvenile and lack the characters required for identification or may be an entity not included in the key.

All Remaning Entities Disappear

There may be occasions when the selection of a particular suite of character states result in all entities disappearing from the “Entities Remaining” quadrant of the key window. In such a case no entity in the key has been scored for the combination of character states selected. This could be attributed to a number of reasons including; user-error in selecting appropriate character states or the specimen in question may not be included in the key. The user may attempt to discover a potential error by sequentially de-selecting all dubious character states. The “Entities Remaining” quadrant may re-populate as character states are removed from the “Features Chosen” list. Carefully re-examine the specimen in question and only choose features and character states that can be selected with confidence. If all entities continue to disappear from the “Entities Remaining” quadrant after several attempts using different combinations of features and states then it is most likely that the specimen in question is not included in the key. Please remember that this key is limited to identifying only the species included.

If the identification of your specimen is critical, it may be sent to the USDA-APHIS-PPQ-National Identification Services representative:

Packaging and Handling Information
David Robinson
USDA-APHIS National Malacologist
Department of Malacology
Academy of Natural Sciences
1900 Benjamin Franklin Parkway
Philadelphia, PA 19103
Tel. 215-299-1175
Fax: 215-567-7229
Email: or