OCR Text |
Show 1889.] R A R E SPECIES O F PLEXAURIDS. 49 The present species is erect, branching, elegant, not at all closely bushy; there is no spreading base, but the trunk is rather wide, being about 3 cm. in diameter ; the branches are flattened at the points of bifurcation; the secondary trunks give rise to branches, which do not ordinarily branch more than five times, so that many of the terminal twigs are as much as 35 cm. long ; these terminal twigs are about 5 m m . in diameter. There are a few nodosities. The cortex is fairly thick, the calyces numerous, irregularly scattered, rather large pits, so that the general appearance is not unlike that of P. porosa. Axis brownish, impregnated with calcareous salts, and the species belongs, therefore, to the genus Plexaurella and not to Plexaura. The characteristic spiculation is shown in the accompanying figure. Hab. West Indies. 5. PLEXAURELLA VERMICULATA. (Plate III. fig. 5.) Gorgonia vermiculata, Lamk. Plexaura vermiculata,\-a\.; see Milne-Edwards & Haime, Corall. i. p. 156. Plexaurella vermiculata, Kolliker, Icon. Histiol. p. 138. There is not, I think, sufficient reason for distinguishing as the type of a distinct species a specimen labelled by Dr. Gray as Plexaura porosa, but which is certainly a Plexaurella, inasmuch as it has deposit of calcic carbonate in its axis. It presents some slight points of difference from P. vermiculata, as described by Milne-Edwards and Haime, and there is not an absolute identity of spiculation. There cannot, I think, be any doubt that there has been a tendency to neglect the variations possible in these forms, and it is better to refrain from " splitting " species. In the specimen under consideration the calices are nearly as closely packed as in P. porosa; the terminal branches are often rather less than 8 m m . in diameter, and the main trunks are from 12 to 16 m m . across, whereas Milne-Edwards gives 8 m m . for the former and 9 to 10 m m . for the latter; no statement, however, is made as to the height of the whole mass; in the case of the British Museum specimen it is rather more than 60 cm. and all the tops are a little broken off. The cortex, which is thick, is of a lightish brown colour ; the axis is of a light brown colour. The axis is flattened at the angles of division, and there appears to be no tendency to the formation of nodes; some of the terminal branches are more than 15 cm. long. The spicules are, as the figures show, short, but somewhat irregular in form. EXPLANATION OF PLATE III. Spicules of Plexaura and Plexaurella, X 180. Fig. 1. Plexaura princi'palis, p. 47. 2. Plexaura suffruticosa, p. 48. 3. Plexaurella affinis, p. 48. 4. Plexaurella anguiculoides, p. 48. 5. Plexaurella vermiculata, p. 49. PROC. ZOOL. SOC-1889, No. IV. 4 |