Abbott’s American Seashells (1974) is the key reference I use to start all my explorations of taxa I find in Florida (keep in mind that my context is principally specimens found in Florida and may not apply to variations occurring very far afield). Then I go to Malacolog, World Register of Marine Species (WoRMS) and Jaxshells (Harry Lee) for current taxonomy. These sources do not always agree (currency and differences of opinion). Because Abbott and several “popular” shell identification books published since 1974 that closely follow Abbott are used by many collectors, my presentations tend to attempt to address taxonomic changes since Abbott, reconciling his taxonomic assignments with current assignments as represented by Malacolog and WoRMS. This presentation is an example that addresses the following:

Mitrella ocellata (Gmelin, 1791)
Mitrella cribraria (Lamarck, 1822)
Mitrella argus (d'Orbigny, 1847)
Mitrella dichroa (G. B. Sowerby I, 1844)

Abbott presented M. ocellata as a valid taxon with M. cribraria as a synonym. He also presented M. argus as a valid taxon with M. dichroa as a synonym. Current taxonomy has M. ocellata as a valid taxon with both M. cribraria and M. argus as synonyms. M. dichroa is presented as a valid taxon. This change is quite significant since many publications that presented M. argus as a valid taxon (often with M. dichroa noted as a synonym) included descriptive narrative and illustrations that apply to M. dichroa and not M. ocellata (or its junior synonyms M. cribraria and M. argus). Therefore, collectors unaware of this may be confused and probably have many misidentified shells. I hope to provide some clarification with this presentation. There are a few pretty reliable characters that distinguish M. dichroa and M. ocellata.

M. ocellata as an adult is almost always severely decollate. M. dichroa is not. M. ocellata is larger as an adult, even when decollate (to 13 mm). M. dichroa (to 8.5 mm). M. ocellata, though somewhat variable, is far more consistent in color/pattern, especially within the same geographic population (background color dark black-brown with numerous small white dots which may be quite large just below the suture). M. dichroa is quite diverse in color/pattern, including within the same geographic population (background color white with brown to orange longitudinal stripping with variations extending to a brown background with very numerous white dots). In mature shells, the columella of M. ocellata is smooth, but the columella of M. dichroa is slightly denticulate.

Though not usually present in specimens in many collections, the protoconch and opercula also differ as noted in the below slides.

References

1. Abbott, R. Tucker. 1974. American Seashells, Second Edition, Van Nostrand Reinhold Company.
2. d'Orbigny, A. in Sagra 1853. Mollusques Histoire Physique, Politique et Naturelle de l'île de Cuba 2. Bertrand, Paris.
3. jaxshells.org
4. Kaicher, S. 1973 to 1992. Kaicher's Card Catalogue of World-Wide Shells.
5. Lee, Harry G. 2009. Marine Shells of Northeast Florida. Jacksonville Shell Club, Jacksonville, Florida.
6. Radwin, George E. April 1978. The Family Columbellidae in the Western Atlantic Part IIa. – The Pyreninae. The Veliger 20(2): 119-133.
7. Rosenberg, G. 2009. Malacolog 4.1.1: A Database of Western Atlantic Marine Mollusca. [WWW database (version 4.1.1)] URLhttp://www.malacolog.org/.
8. WoRMS Editorial Board (2017). World Register of Marine Species. Available from http://www.marinespecies.org at VLIZ. Accessed 2017-01-20. doi: 10.14284/170