OCR Text |
Show 1881.] PROF. F. J. BELL ON THE GENUS ASTERIAS. 503 The more we get into subdivisions the more unsatisfactory will, is obvious, our characters get; and I fully recognize the difficulty, nay, impossibility, of naturalists having any thing like a unanimous opinion on the value of the groups now to be proposed. I give them, however, for what they are worth. They depend on the characters of the spines on the abactinal surface; and, according as these are simple or rare, blunted or acute, we may get divisions to which we may apply the epithets of simplices, rarispinosa?, obtusispinosa?, acutispinosa?. This unsatisfactory method of division is, at present, the only one which can be suggested for the Monacanthid Pentactinida. "When applied in tabular form, we get the following as the result of our attempt:- Genus ASTERIAS. Div. I. HETERACTINIDA. Species with, as a rule, more than five rays. A. POLYPLACIDA. Species with more than one madreporic plate. i. MONACANTHIDA. Adambulacral spines in a single longitudinal row. a. ECHINOPLACIDA. Madreporic plate with a circlet of spines. 0. ft. ANECHINOPLACIDA. No circlet of spines to madreporic plate. (i.) Autacanthida. Dorsal spines placed on special plates. A. tenuispina. (ii.) Typacanthida. Dorsal spines not placed on special plates. A. calamaria. ii. DlPLACANTHIDA. Ambulacral spines in a double row. a. ECHINOPLACIDA. 0. ft. ANECHINOPLACIDA. (i.) Autacanthida. 0. (ii.) Typacanthida. A. acutispina. A. polyplax. A. capensis. |