Page:Scientific results HMS Challenger vol 18 part 1.djvu/699

Rh 6. Axodiscus octogonus, n. sp.

Ring circular or nearly octagonal, connected with the central chamber by eight radial beams at nearly equal distances, which are prolonged outside into eight short conical spines. (Differs from Archidiscus octoniscus in the marginal prolongation of the eight beams.)

Dimensions.—Diameter of the ring 0.04, of the central chamber 0.013.

Habitat.—Central Pacific, Station 274, depth 2750 fathoms.

7. Axodiscus spinosus, n. sp.

Ring circular, connected with the central chamber by ten to twelve radial beams at nearly equal distances, which are prolonged outside into short conical spines of variable length. (May be regarded as an aculeate variety of Archidiscus polythalamus.)

Dimensions.—Diameter of the ring 0.04 to 0.05, of the central chamber 0.014.

Habitat.—Central Pacific, Stations 265 to 274, depths 2350 to 2950 fathoms.

Definition.— without radial appendages of the disk (solid spines or chambered arms on the margin), and without peculiar oscula on the margin of the disk, which is composed of two to four or more concentric rings.

Definition.— with simple circular disk, composed of several rings (without radial appendages or peculiar oscula on the margin of the disk).

The genus Porodiscus is, next to its ancestral form, Archidiscus, the simplest and most primitive form of the Porodiscida, from which all other genera of this family can be derived. The disk is quite simple, without any marginal appendages, composed of a variable number of rings, commonly of circular form, sometimes more or less polygonal, elliptical, or irregular. In my Monograph (1862, pp. 491, 513) I had separated the species, here united in Porodiscus, into two different genera: Trematodiscus with concentric rings, and Discospira with spiral rings. But the extended study of these very common forms in a great number of specimens in the Challenger collection has convinced me that the separation of those two genera cannot be maintained. In one and the same locality, where one single characteristic disk-form is very common, we find intermingled quite regular disks with only concentric, circular rings (Trematodiscus), and other disks with one single perfect spiral ring (Discospira); and between