Page:Scientific results HMS Challenger vol 18 part 1.djvu/525

Rh 4. Prunocarpus melocactus, n. sp.

Cortical shell thick walled, with irregular, roundish pores, twice to five times as broad as the bars; eighteen to twenty-four on the half equator. Between them arise over the entire surface small conical thorns, not longer than the largest pores. Irregularly scattered over the surface twenty to thirty strong, conical, radial spines, about half as long as the main axis, as broad at the base as a large pore. Both medullary shells ellipsoidal, their main axis identical with that of the cortical shell.

Dimensions.—Major axis of the outer shell 0.18, of the middle 0.09, of the inner 0.05; minor axis of the first 0.14, of the second 0.07, of the third 0.04; pores of the cortical shell 0.006 to 0.02, bars 0.004; length of the spines 0.1, basal breadth 0.01.

Habitat.—Pacific, central area, Station 268, depth 2900 fathoms; also fossil in the Tertiary rocks of Sicily; Caltanisetta, Haeckel.

Definition.— with four or more concentric shells (two medullary and two or more cortical shells), without spines or polar tubes.

The genus Cromyodruppa is characterised by the multiplication of the concentric fenestrated shell, which is composed of two medullary shells (enclosed in the central capsule) and two or more cortical shells (outside it). The former may be either spherical or ellipsoidal. The latter are always ellipsoidal, and in this it differs from Cromyosphæra. Probably Cromyodruppa has arisen from Prunulum by secondary apposition of more cortical envelopes.

Definition.—Shell composed of four concentric shells, two medullary and two cortical.

1. Cromyodruppa cepa, n. sp.

Shell composed of two ellipsoidal, cortical, and two spherical medullary shells. Proportion of the main axes of the four shells = 1 : 2 : 4 : 5. Network of all four shells nearly of the same form, subregular, with circular pores of almost equal size in every shell. The absolute size of the pores increases from the innermost to the outermost shell, but the breadth of the bars does not increase in a similar degree. The bars of the outermost shell are only twice as broad as those of the innermost; but the pores are three to four times larger. Surface of all four shells smooth. (The shell greatly resembles that of Cromyatractus tetraphractus, Pl. 15, fig. 2, but is devoid of polar spines; it differs also in the spherical form of both medullary shells and their relative size.)