Page:Scientific results HMS Challenger vol 18 part 2.djvu/125

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Definition.— with two bisected horizontal rings, which are connected by three vertical meridional rings (or six columellæ).

The genus Tympaniscus and the following Tympanidium differ from all preceding Tympanida in the multiplication of the vertical columellæ, which connect the two horizontal bisected rings. Tympaniscus exhibits six columellæ, which may be regarded as halves of three meridional rings. Only one, however, of these is complete (the sagittal ring, which bisects the mitral and the basal ring); the two other meridional rings (placed in diagonal planes) may be regarded as produced by lateral bifurcation of an incomplete frontal ring; this appears to be already foreshadowed in the preceding Octotympanum.

1. Tympaniscus corona, n. sp.

Basal ring larger than the mitral ring, with numerous irregular short spines but without descending feet. Six columellæ strongly curved, in the lower half thicker and thorny; the four lateral much thinner than the two sagittal. Apex of the jointed sagittal ring with a conical horn.

Dimensions.—Length of the shell 0.08, breadth 0.11.

Habitat.—Central Pacific, Station 272, depth 2600 fathoms.

2. Tympaniscus bipes, n. sp.

Basal ring of about the same size as the mitral ring, with two lateral descending feet, which are parallel and vertical, simple, without spur, half as long as the sagittal ring. The latter is ovate, twice as long and broad as the two circular frontal rings.

Dimensions.—Breadth of the shell 0.14, length 0.09.

Habitat.—North Pacific, Station 244, depth 2900 fathoms.

3. Tympaniscus dipodiscus, n. sp. (Pl. 94, figs. 5, 6).

Basal ring smaller than the mitral ring, with two lateral descending feet, which are nearly parallel and vertical, obtuse, about as long as the sagittal ring, and which bear on the outside a horizontal spur. Sagittal ring hexagonal, very stout, with a caudal appendix at the base, and with a small sagittal arch, bearing a stout spinulate double knob at the apex. The two subfrontal rings depressed, twice as broad as high, on the outside thorny, connected at each side in the lower half by a double transverse arch.

Dimensions.—Breadth of the shell 0.18, length 0.08 (with spines 0.18).

Habitat.—Central Pacific, Station 268, depth 2900 fathoms.