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Show 502 PROF. F. J. BELL ON THE GENUS ASTERIAS. [May 3, iii. POLYACANTHIDA. panopla. The number of species in the first two groups of the Pentactinida is still so large that we must now attempt to find some means by which they mav be still further subdivided. Among a certain number of forms we find a special modification of some of the dorsal spines, which come to form an encircling fringe around the madreporic plate. In some species these spines are obvious enough ; but I know of no author except Prof. Verrill who has directed any attention to them1, or attached any importance to their presence. As, however, I will show in detail further on, I cannot give in m y adhesion to the validity of the genus Lep-tasterias, even when the proposition comes from so deservedly honoured and distinguished a naturalist. I doubt, in fine, whether they can be used as any thing more than a convenient separation-character in a genus where specific characters are so rare. The species, then, which are provided with a circlet of spines round their madreporic plate may be distinguished as the Echinoplacida; such among the Diplacanthid Pentactinida are:- A.fulva (with 18 spines). A. germaini (with spines indistinct). A. lurida (with 12 spines). A. nuda (with spines irregularly distributed). A. obtusispinosa (with 12 spines). A. sinusoida (with spines irregularly distributed). A. spectabilis (with 18 spines). It is curious to observe that, as yet, the echinoplacid condition not been noted as obtaining among the Monacanthid Pentactinida; among the Polyactinida the echinoplacid condition seems only to have been observed in the diplacanthid monoplacid form acervata, where there are said to be 13 spines around the madreporic plate. In a description of the species A. brandti, which I lately communicated to the Societv2,1 have directed attention to the mode of arrangement of the greater number of the intermediate spines on special local modifications of the integument, which may be known as special plates. This arrangement should be familiar enough ; for it is to be found in A. tenuispina, and is represented in the figure of that species given by Muller and Troschel (' System der Aster.' pi. i. fig. 1 b). The forms in which the spines thus rise from special plates may be distinguished as autacanthid. In such a group we should find:-the Polyactinid meridionalis, perrieri, tenuispina, and (probably) scabra; and the Pentactinid Diplacanthid brandti and neglecta. When the spines retain the simpler disposition which is seen in A. rubens and most of the better known forms, we may speak of the arrangement as being typacanthid. 1 Of course such a naturalist as Philippi does not fail to note their presence. 2 P. Z. S. 1881, p. 91. |