with the apex slightly beaked and without a longitudinal furrow. Shell and cup-fbrmed basis 

 covered by a tissue or film of the Alcyonarian to which it is attached. 



This remarkable species presents us with an interesting case of commensalism. The 

 cup-fonned basis is attached to a part of the bifurcating stem of an Acanthogorgia, which is 

 covered by a substance composed of a tissue interwoven with calcareous spiculae and developing 

 here and there into little calyces, from the surface of which numerous spiculae stand off (PI. 

 XXIII, fig. 4). The shell of the Balanus is covered by the same substance which here also 

 develops into numerous such calyces. 



This Balanus belongs to the same section of the genus as B. calceohis, B. navicula 

 and others. lts shape (PI. XXIII, fig. 5) is conical, the shell being elongate, however, in its 

 rostro-carinal axis. In both specimens it is the rostrum that, by its protraction, causes the 

 elongation of the shell. The orifice is rather large and rounded, somewhat heart-shaped owing to a 

 little excavation at the rostral extremity. The carinal-latus is relatively narrow, the latus triangular, 

 with the summit horizontally truncated. The radii are narrow, with the summits somewhat 

 oblique, the alae have the summits nearly parallel to the basis. The basis is cup-formed, much 

 elongated towards the rostral extremity of the shell; in the median line the rostral part of the 

 basis has a deep furrow, which quite encloses the stem of the colony to which the Balanus is 

 attached. The colour is pale-yellowish, that of the basis transparent white; the shell is very 

 thin and brittle, and the compartments readily separate. 



The scutum (PI. XXIII, fig. 6, a and c) is rather thick and has very distinct growth- 

 ridges. Its basal margin is distinctly sinuous. The articular ridge is prominent, the adductor 

 ridge slightly, yet distinctly developed ; the cavity for the adductor muscle is indicated. 



The tergum (PI. XXIII, fig. 6, b and d) has the apex slightly beaked, the articular 

 ridge well-developed, and the spur rather broad, and obtusely pointed. There is no longitudinal 

 furrow, but in its place the surface developes into a kind of longitudinal crest. The crests for the 

 depressor muscles although not prominent are distinctly indicated. The scutal margin is nearlv 

 straight, the basal margin, between the spur and the basi-carinal angle, distinctly hollowed out. 



With regard to the structure of the animal's body the following may be of interest: 



Mouth. Labrum (PI. XXIII, fig. 7) with rather deep notch, wide at entrance, closed 

 near the bottom. Three small teeth on each side, one of these, however, is wanting in one 

 of the specimens. The lateral margins on each side of notch not quite in a line but together 

 making an angle. The sub-triangular shield-like thickening is about as long (or high) as it 

 is broad. 



Pal pi ovate, with the upper margin straight, and the lower margin distinctly convex, 

 the two margins meeting each other in a rounded angle. Spines and hairs on surface not 

 numerous; those along the upper margin short; a group of from four to half-a-dozen longer 

 hairs are disposed on the outer surface along a line forming an angle with the lowor margin 

 near the free extremity. The free extremity, moreover, bears a few longer hairs forming a tuft 

 with those of the outer surface and the upper margin. 



Mandible (PI. XXIII, fig. 8) has five teeth; teeth 2, 3, and 4 are doublé. Tooth 3 

 has the larger one broad and flat. Distance between extremities of teeth 1 and 2, one and 



97 



SIBOGA-EXPED1TIE XXXI b. 29 



