364 COLLECTIONS FROM MELANESIA. 



37. Psilacabaria gracillirna. 

 (Plate XXXVII. figs. D-D" ; Plate XXXYIII. figs./-/".) 



Corallum extremely delicate, erect, branching dichotomously, 

 approximately in one plane, and at approximately right angles. 

 Apparently no anastomosis. Internodes 12-16 millim. long, diameter 

 •3 to - 7 millim., cylindrical, slightly flexuous. Soft joints incon- 

 spicuous, not broader than the adjacent internodes. Axis of inter- 

 nodes hard, white, not striated. 



Cortex about one quarter the thickness of the whole branch, rough- 

 ened b}' the large spicules. Verrucae arranged in spiral series, with 

 bases almost in contact on larger branches, rather distant and on 

 opposite sides on the smaller ones ; triangular and flattened in the 

 former, low, rounded, and tubercular in the latter ; basal diameter 

 and height 1 millim. in the large to *5 millim. in the small branches. 

 Colour of cortex white, of tips of verrucas pale yellow. 



Spicules of general cortex: — (i.) Large cylindro-fusiform to sub- 

 clavate, with thick axis, slightly tapering to round-pointed ends, well 

 provided with strong tubercles about "03 millim. high, either sub- 

 spherical or forming low roughened ridges often extending round as 

 much as one quarter of the circumference of the spicule (apparently 

 representing tubercular whorls of those spicules of other Helithseidae 

 which possess them) ; size -28 to *31 by *087 millim. ; tubercles 

 commonly more abundant and larger near one end than the other, 

 (ii.) Smaller, fusiform, with sharply pointed ends and prominent 

 pointed and roughened tubercles, more or less curved, scattered 

 abundantly all over; size -21 to "25 by -053 to "071 millim. (also at 

 base of verrucae) [probably young forms of (i.)]. Spicules of verrucae : — 

 (iii.) Similar to (i.), but with less broad and ridge-Kke tubercles, 

 measuring as much as *3 by -087 millim. ; on exterior of the verrucae. 

 except just at the apex, (iv.) Subclavate, tapering from a broad to 

 a sharp-pointed end, well covered with irregularly scattered, pro- 

 minent, pointed tubercles, which become larger towards the broad 

 end, where they project forwards beyond and in the direction of 

 the axis of the spicule ; on the inner wall of the verrucae, thus 

 resembling the subapical verruca-spicules of Acabariu divaricate. 

 (v.) Smaller, fusiform, with fewer tubercles, pointed, scattered 

 equally all over, two or three terminal ones at one end prolonged 

 much as in (iv.) ; size *21 to -25 by "035 to - 044 millim. ; on apex of 

 verrucae [probably young forms of (iv.)]. (vi.) Intermediate forms 

 between (i.) and (ii.), with tubercles more distinct and smaller than 

 in (i.) ; size - 31 by - 07 millim. ; on general body of zooid. 



Of all these six forms I only distinguish 3 distinct types, viz. : — 

 (i.) [including iii. and vi.] representing the large cortical fusiform of 

 other Melithaeidae, but found also on the verruca, from the fact of 

 the latter being covered more or less with what represents the 

 general cortex ; (ii.) representing the smaller cortical fusiform and 

 perhaps also the horizontal verruca-spicule of other Melithaeidae ; 

 (iii.) [including v.] representing the longitudinal verruca-spicule, 



