The extremely rare Periploma (Offadesma) angasai Crosse & Fisher 1864 is a member of the widely distributed Periplomatidae (Anomalodesmata) occurring on surf‐protected beaches in New Zealand and Australia. The species is low intertidal‐sublittoral in distribution, living 7–8 cm below the sand surface on its left valve with separate siphons projecting to the sand‐water interface above. The shell is sharply inequivalve (the right deeply, the left shallowly convex) and inequilateral, the posterior rostrate and truncate and covered by thick periostracum densely studded with sand grains. These are attached to the periostracum by a sticky secretion produced from radial mantle glands in the middle mantle fold. Similar glands have been reported for members of the Lyonsiidae and Verticordiidae. Offadesma angasai is adapted for a life in which large amounts of fine sediment may enter the mantle cavity. Thus the ciliary rejection currents are powerful, the labial palps are extremely long and the sediments are bound up in mucus produced by an extensive gland, particularly in the right mantle lobe, in the posterior region of the mantle cavity. A gland with a similar structure but different staining reactions also lines the visceral mass and the mantle of the supra‐branchial chamber and probably consolidates particles penetrating the ctenidial filter. Such glands are similar in structure and function to the primitive molluscan hypo‐branchial gland. In many respects, especially with regard to the secondary ligament of fused periostracum, a ctenidial structure with type E ciliation, the presence of a 4th pallial aperture, extensive mantle fusions and simultaneous hermaphroditism, Offadesma is clearly similar to the majority of the Anomalodesmata. As in many other anomalodesmatans also, large, yolky, encapsulated eggs are produced; these are thought to be an adaptation facilitating survival by a sparsely distributed species occupying a narrow niche. Many other features of Offadesma, notably the structure of the ligament and shell clearlyally it and the Periplomatidae with the Laternulidae and the Thraciidae in a separate line of evolution from the other families of the Pandoracea, and probably deserving of super‐family status—the Thraciacea Stoliczka 1870.

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{{Explor lien
   |wiki=    Wicri/Santé
   |area=    EdenteV2
   |flux=    Main
   |étape=   Exploration
   |type=    RBID
   |clé=     ISTEX:F09662E1F6991360FBA1642EEDEB219AE9C178DF
   |texte=   The biology and functional morphology of Periploma (Offadesma) angasai (Bivalvia: Anomaiodesmata: Periplomatidae
}}