The genus Rhopalastrum, founded by Ehrenberg (1847) with a very insufficient diagnosis, is here retained for those Trigonastrida that agree in the generic characters with the only species figured by him, viz., Rhopalastrum lagenosum (compare my Monograph, 1862, p. 500). It comprises, therefore, such Euchitonida as agree with the preceding Dictyastrum in the simple form of the three arms and the absence of a patagium, but differ from it in the different size of the three angles, and often also in the divergent form and size of the three arms; one odd arm is opposite to the odd angle between the two paired arms.
Subgenus 1. Rhopalastrella, Haeckel.
Definition.—Arms with blunt ends, without terminal spines.
1. Rhopalastrum truncatum, Haeckel.
Rhopalastrum truncatum, Haeckel, 1862, Monogr. d. Radiol., p. 500, Taf. xxix. fig. 6.
Distance of both paired arms about half as large as their distance from the odd arm. All three arms nearly of the same form and size, very short and broad; their breadth nearly equals that of the roundish central disk, whilst their length reaches only one-fourth of it. End of the arms convex rounded, without spines.
Dimensions.—Radius of each arm 0.1, breadth 0.15.
Habitat.—Mediterranean (Messina), Atlantic (Canary Islands), surface, Haeckel.
2. Rhopalastrum pistillum, Stöhr.
Rhopalastrum pistillum, Stöhr, 1880, Palæontogr., vol. xxvi. p. 110, Taf. v. fig. 4.