The genus Litholophus, the only one of this family, exhibits the peculiarities just described, but might more nearly be defined as a typical "genus" by the quadrangular form of the radial spines, identical with those of Acanthonia.
The central capsule of Litholophus is constantly conical or pyramidal, commonly opaque, of a dark brownish or reddish colour; it contains many small nuclei. It envelops the basal half of all radial spines in such a manner that their basal parts are united in its apex, and their distal parts pierce the rounded base of the conical capsule (Pl. [129], fig. 2).
The calymma or the jelly envelope of the central capsule is only developed at its base, where the spines radiate; at the conical mantle of the capsule it is very thin. The spines seem to be perfectly enclosed in the calymma and connected with it by the same contractile retinacula or "myophrisca" which we observe in the Acanthonida. The pseudopodia arise only from the rounded base of the conical capsule, and radiate between the spines, piercing the calymma, diverging within the conical space occupied by the fascicle of spines.
Subgenus 1. Litholopharium, Haeckel.
Definition.—Ten radial spines.
1. Litholophus decimalis, n. sp.
Ten radial spines, four-sided prismatic, with prominent smooth edges, of equal breadth throughout their whole length.
Dimensions.—Length of the spines 0.2 to 0.3, breadth 0.006.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, surface.
2. Litholophus pyramidalis, n. sp.