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.

<==Arachnoididae D66
    |  i. s.: Fossulaster Lambert & Thiéry 1925 D66
    |           `--*F. halli Lambert & Thiéry 1925 M89 [incl. Scutellina patella Hall 1908 non Tate 1891 D66]
    |--Arachnoidinae D66
    |    |--Arachnoides Leske 1778 M89 [incl. Echinarachnius Leske 1778 (nom. rej.) non Gray 1825 D66]
    |    |    `--*A. placenta (Linnaeus 1758) [=Echinus placenta] M89
    |    `--Fellaster Durham 1955 D66
    |         `--*F. zelandiae (Gray 1855) M89 [=Arachnoides zelandiae M89, Echinarachnius zelandiae D66]
    `--Ammotrophinae D66
         |--Monostychia Laube 1869 D66
         |    `--*M. australis Laube 1869 M89
         |--Scutellinoides Durham 1955 D66
         |    `--*S. patella (Tate 1891) [=Scutellina patella] M89
         `--Ammotrophus Clark 1928 M89 [incl. Hesperaster Clark 1938 D66]
              |--*A. cyclius Clark 1928 M89
              `--*Hesperaster’ arachnoides Clark 1938 D66

*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.

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