Page:Scientific results HMS Challenger vol 18 part 2.djvu/422

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Definition.— (vel Dicyrtida eradiata aperta) with discoidal, flatly expanded thorax. Cephalis large, without horn.

The genus Sethocephalus (formerly proposed under the name Platycryphalus) contains some few and rare Sethocorida, distinguished by the very large cephalis, the collar opening of which is wide open, not closed by cortinar beams, and surrounded by a very flat, nearly discoidal thorax, like the brim of a hat. It is possible that this peculiar genus has been derived from a Tricyrtid (Theocalyptra?) by loss of the original cephalis, and that the apparent large cephalis is the original thorax.

1. Sethocephalus eucecryphalus, n. sp. (Pl. 56, fig. 13).

Cephalis very large, campanulate-conical, with irregular, polygonal pores and thin bars. Thorax short, scarcely broader than the cephalis, beyond the sharp collar stricture expanded like the brim of a hat, with few rows of irregular, polygonal pores. (In the specimen figured the thorax was only half as broad as in another specimen found afterwards.)

Dimensions.—Cephalis 0.12 long, 0.09 broad; thorax 0.02 long, 0.12 broad.

Habitat.—South Pacific, Station 285, surface.

2. Sethocephalus platycryphalus, n. sp.

Platycryphalus sethodiscus, Haeckel, 1881, Prodromus, p. 430.

Cephalis large, hemispherical, with irregular square pores and thin bars. Thorax flatly conical, three to four times as broad as the cephalis, beyond the collar stricture expanded nearly discoidally, with a delicate network of small, subregular, hexagonal pores. (Similar in general form to Sethophormis aurelia, Pl. 55, fig. 3, but without any trace of collar beams or radial ribs.)

Dimensions.—Cephalis 0.04 long, 0.08 broad; thorax 0.04 long, 0.3 broad.

Habitat.—South Pacific, Station 289, surface.

Definition.— (vel Dicyrtida eradiata aperta) with ovate or subcylindrical thorax, the mouth of which is constricted, simple, without prominent hyaline peristome. Cephalis with an apical horn.