Page:Scientific results HMS Challenger vol 18 part 2.djvu/38

914 2. Plagonidium quadrigeminum, n. sp.

Spines cylindrical, curved, eight to ten times as long as the common central rod, in the distal half forked; the fork-branches curved, somewhat longer than the basal part.

Dimensions.—Length of the spines 0.18, of the middle rod 0.02.

Habitat.—Antarctic Ocean, Kerguelen Island, Station 159, surface.

Definition.— with four unequal radial spines, arising in pairs from the two poles of a common central rod; one ascending apical spine opposed to three descending basal spines.

The genus Plagiocarpa agrees with the preceding Plagonidium in the possession of a common horizontal middle rod, the two poles of which bear two pairs of divergent spines; but whilst in the preceding all four spines are equal, here they are differentiated in the same manner as in Plagoniscus, which differs only in the absence of the middle rod. The two observed and closely allied species of this genus are of peculiar interest, since they belong possibly to the common ancestral forms of the ; the basal middle rod corresponds perhaps to the basal part of a sagittal ring, the apical spine to its dorsal part, the three other spines to the basal feet (compare above, p. 902).

1. Plagiocarpa procortina, n. sp. (Pl. 91, fig. 5).

Spines cylindrical, curved, thorny, three to four times as long as the common middle rod. Apical spine and the meeting caudal spine (or the posterior basal spine) somewhat longer and with more numerous thorns than the two paired pectoral (or anterior) spines. From the common base of the latter arises an anterior prolongation of the horizontal middle rod, which in the sagittal plane is curved upwards and corresponds to the sternal foot of many. An ascending branch of this spine is opposed to a descending branch of the apical spine, both together forming an incomplete sagittal ring.

Dimensions.—Length of the two larger spines 0.15, of the two smaller 0.12.

Habitat.—Mediterranean (Portofino near Genoa), surface.

2. Plagiocarpa procyrtella, n. sp.

Spines of form and arrangement similar to those of the preceding species, but longer and more branched, six to eight times as long as the shorter common middle rod. The two characteristic