Diloma
A genus of medium-sized marine snails in the family Trochidae.
Diloma is a genus of medium-sized marine gastropod mollusks belonging to the family Trochidae, commonly known as top snails. (A separate genus of phytoplankton, Diloma F.H.Wind & P.Cepek, 1979, exists in the class Prymnesiophyceae.)
The shell is solid, imperforate, and depressed-globose in shape. Its color is typically slate or black, though worn specimens may appear reddish or brownish. The spire is short and conical, with an acute apex that is usually reddish. The sutures are linear, and the five whorls are slightly convex, increasing rapidly and marked with obsolete spiral striae. The body whorl is often depressed or slightly concave below the suture. The base is rounded, eroded, and iridescent near the aperture. The aperture itself is large, oblique, and iridescent. The outer lip is rather thin, not black-margined inside, but bordered by a brilliantly iridescent band. The columella is concave, with an obsolete subdentate lower portion, and is very broad and flattened or excavated on its face; it is composed mainly of an opaque white layer that also lines the base but does not extend to the lip edge. Shell length ranges from 15 mm to 26 mm, and diameter from 17 mm to 24 mm.
This genus is distributed across the Indo-Pacific region, including New Zealand, Japan, and other areas.
Species include: *Diloma aethiops*, *D. amoenum*, *D. aridum*, *D. bicanaliculatum*, *D. concameratum*, *D. coracinum*, *D. durvillaea*, *D. filipino*, *D. hsiujuae*, *D. imitationis*, *D. lepidum*, *D. miocenica* †, *D. nanum*, *D. nigerrimum*, *D. oriens*, *D. piperinum*, *D. radula*, *D. samoaense*, *D. subrostratum*, *D. sulci*, *D. triumtrium*, *D. yanliaoense*, and *D. zelandicum*. Several former species have been moved to synonymy, including *Diloma arida* (syn. *D. aridum*), *D. constellatum* (syn. *Austrocochlea constellata*), *D. gaimardi* (syn. *D. aethiops*), *D. impervium* (syn. *Oxystele impervia*), *D. nana* (syn. *D. nanum*), *D. plumbea* (syn. *D. subrostratum*), *D. sinense* (syn. *Oxystele sinensis*), *D. suavis* (syn. *Pictodiloma suavis*), *D. tabulare* (syn. *Oxystele tabularis*), *D. tigrinum* (syn. *Oxystele tigrina*), *D. variegatum* (syn. *Oxystele antoni*), *D. subrostratum novaezelandiae* (syn. *D. subrostratum*), *D. (Chlorodiloma) millelineata* (syn. *Chlorodiloma millelineata*), and *D. (Fractarmilla) lenior* (syn. *D. bicanaliculatum*).
Quick Facts
- Taxon
- Diloma
- Type Species
- Turbo nigerrimus
- Type Species Authority
- Gmelin, 1791
Facts from the source article.
Lore & Background
The shell of Diloma is solid, imperforate, and depressed globose, with a short conic spire and an acute, usually reddish apex. The sutures are linear, and the five whorls are slightly convex, rapidly increasing, and spirally obsoletely striate. The body whorl is usually depressed or subconcave below the suture, and the base is rounded, eroded, and iridescent in front of the aperture. The aperture is huge and oblique, with a thin outer lip bordered by a brilliantly iridescent band, not black-margined within. The columella is concave, obsoletely subdentate below, very broad and flattened or excavated on the face, composed principally of an opaque white layer that also lines the base but does not extend to the edge of the lip.
Reader's Guide
The genus Diloma is significant within the family Trochidae for its distinctive shell morphology, including a large iridescent aperture and a characteristic columella structure. The article lists numerous species, such as Diloma aethiops, Diloma bicanaliculatum, and Diloma zelandicum, among others, indicating a diverse genus with a wide Indo-Pacific distribution. Several species have been brought into synonymy, reflecting taxonomic revisions. The genus also shares its name with a genus of phytoplankton in the class Prymnesiophyceae, but the marine snail genus is the focus here. The article provides a partial phylogram and a list of species, including recently described ones from 2026, showing ongoing research. Its legacy lies in its contribution to the understanding of trochid diversity and evolution in the Indo-Pacific region.
Did You Know?
- The shell of Diloma varies in length from 15 mm to 26 mm and in diameter from 17 mm to 24 mm.
- The genus occurs in the Indo-Pacific, including New Zealand and Japan.
- The aperture is huge, oblique, and iridescent, with a brilliantly iridescent band bordering the outer lip.
- There is also a genus Diloma in the class Prymnesiophyceae, a group of phytoplankton.
Frequently Asked Questions
What is Diloma?
Diloma is a genus of medium-sized sea snails classified within the top snail family, Trochidae. It is a marine gastropod mollusk that inhabits waters across the Indo-Pacific region.
What does a Diloma shell look like?
The shell is solid with no basal opening, rounded and somewhat flattened in overall shape, and typically slate-gray or black in color. It features a short conical spire with a reddish tip and five slightly bulging whorls that grow rapidly in size, marked by faint spiral lines.
How big does Diloma get?
Adult shells measure roughly 15 to 26 millimeters in length and 17 to 24 millimeters in diameter. This places the genus squarely in the medium-sized range for top snails.
Where can I find Diloma in the wild?
This genus is distributed across the Indo-Pacific, with records from New Zealand, Japan, and other areas within that broad marine zone. It is a coastal or shallow-marine species rather than a deep-water one.
How is Diloma different from other top snails?
Diloma is set apart by its solid, imperforate, depressed-globose shell shape combined with a slate or black body color and a distinctly reddish spire apex. Its five whorls increase rapidly in size and bear only obsolete spiral striae, giving it a smoother look than many congeners in Trochidae.
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