REPORT ON THE ALCYONARIA. 137 



Genus Plexaura, Lamouroux, emend. 



Plexaura, Lamouroux, Hist, des Polypiei-s flexibles, p. 24, 1816. 



„ Milne-Edwards, Hist. Nat. des Coralliaires, vol. i. p. 152, 1857. 

 „ Kolliker, pro ^ari'', Icon. Histio)., Abth. ii. Heft 1, p. 138, 1865. 



Plexaura valenciennesi, n. sp. (PI. XXXIII. fig. 1), 



Plexaura flexiiosa, Lamk., Valenciennes ("de la Havane"), Comptes rendus, t. 41, p. 12, 1855. 

 Xon „ „ Lamouroux, Hist, des Polypiers flexiVjies, p. 424, 1816. 



The species of this genus wouki appear to be essentially shallow-water forms. Only 

 a single species appears in the Challenger collection ; this was dredged in one of the 

 Bays of the Bermudas, in quite shallow water. It appears to be as yet undescribed, 

 although it is the form referred to by Valenciennes which is in the Museum at Paris. 



In the only specimen preserved the colony arises from an attached and somewhat 

 spreading base to a height of 24 cm., the main axis being at its base 1 2 mm. in diameter ; 

 when about 12 mm. high it branches. The larger of the two branches is 8 mm. in 

 diameter and attains a height of 20 cm.; from it several other branches proceed, but 

 from the one side only and in the same plane, and from these secondary twigs proceed in 

 like manner. 



The diameter of the principal twigs is from 3 to 5 mm. The basal portion of the 

 axis is denuded of polyps, and here the outer layer of purple spicules being worn off, the 

 middle layer of large spicules is seen to form a compact pavement. 



The colour of this portion of the axis is a dirty brown ; that of the upper portion and 

 of the branches is a dark purple with a slight tinge of yellow. 



The polyps are numerous, thickly set in irregular spirals round the stem and branches, 

 extending to the very summits of these latter. The position of each polyp is very 

 distinctly marked by a short papilliform projection which is often circular but sometimes 

 oval, and within which the polyp is completely retractile, the tentacles being drawn in 

 last, when the central pit slightly contracts, and in doing so a bilabial appearance is 

 sometimes seen. These projections rise but slightly from the level ol the ccenenchyma, 

 but sufficiently so to give a quite pustulate appearance to the colony. 



The ccenenchyma is massive, and formed of several layers of spicules ; the outer layer 

 consists of purple-coloured spicules, foliated clubs, and spindles, beneath which is a layer 

 of large, tuberculate, fusiform spicules, while the innermost layer is formed of small, 

 radiate, tripartite and fusiform spicules of a bright purple colour or colourless. 



The small nutrient canals from each polyp penetrate beneath the first layer in 

 communicating with those of the neighbouring polyps, while a series of larger nutrient 

 canals runs parallel with the axis within cover of the middle layer of spicules. The 

 central axis is horny. 



The polyp bodies, at least the retractile portions, are destitute of spicules, uidess for 



(ZOOL. CHALL. ZXP. — PART LXIV. 1888.) SsS 18 



