Oral view of Ammotrophus cyclius, from here.

Belongs within: Clypeasteroida.

The Arachnoididae are a group of sand dollars known from the Oligocene to the present, in which the accessory tube feet outside the petals are usually arranged in a characteristic ‘comb’ pattern (Durham 1966).

Characters (from Durham 1966): Test flattened, outline usually rounded; ambitus moderately thin; petals open; ambulacral food grooves simple, well defined, no secondary tube feet in grooves; accessory tube feet outside petals usually in dense oblique series (“combs”), restricted to ambulacral areas; four genital pores; peristome not sunken; buccal membrane plated; primordial interambulacral plates externally larger than ambulacral plates.

    |    |--Arachnoides Leske 1778M89 [incl. Echinarachnius Leske 1778 (nom. rej.) non Gray 1825D66]
    |    |    |--*A. placenta (Linnaeus 1758) [=Echinus placenta]M89
    |    |    `--A. tenuis Clark 1938SM15
    |    `--Fellaster Durham 1955D66
    |         `--*F. zelandiae (Gray 1855)M89 [=Arachnoides zelandiaeM89, Echinarachnius zelandiaeD66]
         |--Monostychia Laube 1869D66
         |    `--*M. australis Laube 1869M89
         `--Ammotrophus Clark 1928M89 [incl. Hesperaster Clark 1938D66]
              |--*A. cyclius Clark 1928M89
              `--*Hesperaster’ arachnoides Clark 1938D66

*Type species of generic name indicated


[D66] Durham, J. D. 1966. Clypeasteroids. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt U. Echinodermata 3 vol. 2 pp. U450–U491. The Geological Society of America, Inc., and The University of Kansas Press.

[M89] Mooi, R. 1989. Living and fossil genera of the Clypeasteroida (Echinoidea: Echinodermata): an illustrated key and annotated checklist. Smithsonian Contributions to Zoology 488: 1–51.

[SM15] Sampey, A., & L. M. Marsh. 2015. Kimberley marine biota. Historical data: echinoderms. Records of the Western Australian Museum Supplement 84: 207–246.

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