Definition.—Druppulida with four or more concentric shells (two medullary and two or more cortical shells), with numerous radial spines, but without polar tubes.
The genus Cromyocarpus differs from Cromyodruppa only in the development of numerous large radial spines which start from the outer surface of the shell. It exhibits therefore the same relation to the latter that Prunocarpus bears to Prunulum, or Druppocarpus to Druppula. It differs from both in the multiplication of the cortical shell.
1. Cromyocarpus quadrifarius, n. sp.
Shell composed of four concentric shells, two ellipsoidal cortical shells, and two spherical medullary shells. Proportion of their main axes = 10 : 8 : 3 : 2. Pores of the two cortical shells irregular, roundish, twice to four times as broad as the bars. Pores of the two medullary shells subregular, circular, about the same size as the bars. Outer surface covered with numerous conical radial spines, about half as long as the main axis, half as broad at the base as the innermost shell.
Dimensions.—Major axis of the outer cortical shell 0.2, of the inner 0.15; equatorial axis of the former 0.16, of the latter 0.12; diameter of the outer medullary shell 0.06, of the inner 0.04; pores of the cortical shells 0.01 to 0.02, of the medullary shells 0.004 to 0.008, bars 0.003 to 0.006; length of the radial spines 0.1, basal breadth 0.02.
Habitat.—Indian Antarctic Ocean, Station 157, depth 1950 fathoms.
Genus 137. Lithatractus,[[176]] n. gen.
Definition.—Druppulida with simple ellipsoidal cortical shell and simple medullary shell, with two large opposite polar spines in the main axis of equal size and similar form.
The genus Lithatractus, rich in common and widely distributed species, begins the series of those Druppulida which are characterised by peculiar polar spines at both poles of the main axis. It repeats the formation of Stylosphæra and Ellipsostylus, and differs from the former in the ellipsoidal form of the cortical shell, from the latter in the possession of a medullary shell. Formerly all these forms were united in the one genus Stylosphæra (see above, p. [121]).