The genus Halicalyptra and the two following closely allied genera differ from the preceding Archiphormida in the absence of radial ribs. The multiradiate structure is indicated only by the terminal spines or feet, forming a coronet around the mouth. These feet are either the free ends of reduced ribs, or new productions of the peristome. Halicalyptra may be derived from Calpophæna and Petalospyris by loss of the basal lattice-plate of the cephalis. But it is also possible that it has been derived from Anthocyrtis by loss of the cephalis, and that its shell corresponds to the thorax of the latter.
Subgenus 1. Acrocalpis, Haeckel, 1881, Prodromus, p. 427.
Definition.—Shell smooth, without spines or thorns (other than the feet and the apical horn).
1. Halicalyptra virginica, Ehrenberg.
Halicalyptra virginica, Ehrenberg, 1854, Mikrogeol., Taf. xviii. fig. 110.
Halicalyptra virginica, Haeckel, 1862, Monogr. d. Radiol., p. 289.
Haliomma virginicum, Ehrenberg, 1844, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 71.
Acrocalpis virginica, Haeckel, 1881, Prodromus, p. 427.
Shell smooth, campanulate, nearly hemispherical. Pores subregular circular, about as broad as the bars. Horn conical, half as long as the shell. Peristome scarcely constricted, with six conical divergent feet, as long as the horn.
Dimensions.—Shell diameter 0.04 to 0.05, length of the feet 0.02.