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Show 1900.] MR. F. P. BEDFORD ON MALAYAN ECHINODERMS. 273 mouths of the rivers extensive deposits of silt mixed with vegetable debris occur, and here, as a rule, the reef is poorly, if at all, represented; in places, however, where the reef is flourishing its distance from the shore is very variable, rarely exceeding 500 yards, and the lagoon shore is of the most varied description-mangrove-swamps, tracts of hard sand, or gravel may occur, or volcanic rocks may come down to the water's edge and, being eroded by the action of the sea, form tide-pools and crevices when the tide recedes. Each kind of environment was found to have its own peculiar Echinoid fauna, each species except the most abundant being restricted to one sort of habitat in its adult condition; and in this connection it is worth noting that, although this is constant throughout the district, yet in other localities, e. g., Batavia, the same species will be found in a different environment. A good instance of this is afforded by Diadema saxatile, which was the only reef-Echinoid of the district, and was a conspicuous object among the masses of coral on nearly all the reefs ; whereas at Batavia this species was apparently found by Dr. Sluiter (cf. also Studer *) a short distance from coral on patches of sand, Echinometra lucunter being common among coral in that locality. It seems to m e that, by a similar change of habitat, a certain amount of isolation might be caused which would enable variations in a direction favourable to that environment to become normal for the particular form-unit, and by a continuation of the process specific distinctions might become fixed. Without some such isolation it is difficult to imagine how any variations from the norm could become sufficiently stable to be perpetuated alongside of the type, or how any gradual modification could take place in the type itself 2, since the pelagic plutei that settle down in any given limited area can hardly be supposed to be the offspring of adults living within that same area, and there is no evidence, so far as I am aware, of discontinuous variation among Echinoids likely to lead to the formation of new races. The species of which examples were obtained number sixteen, of which it may be worth remarking that four extend as far south as N e w Zealand; this is interesting in connection with the distribution of Asteroids and Holothurians, which appears to be much more limited. Only a few Echinoids have been previously recorded from the locality, but of these there are two species which we have failed to find, and which are not represented in the Raffles Museum ; they are Salmacis dussumieri and Rhabdocidaris bispinosa; there are specimens of the latter in the British Museum collected by Dr. Powell at Singapore, and I have satisfied myself of their distinctness from Rhabdocidaris annulifera. Only one new species is described, belonging to the morphologically interesting genus Asthenosoma, and we were fortunate in obtaining two young > T. Studer, Monatsber. d. Ak. Wiss. Berlin, 1880, p. 868. 2 Cf. A. E. Ortmann, Grundziige d. mar. Tiergeographie, 1896, p. 31. |
