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with the apex slightly beaked and without a longitudinal furrow. Shell and cup-formed 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-formed 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 Salanus is covered by the same substance which here also 
develops into numerous such calyces. 
This Lalanus belongs to the same section of the genus as #. calceolus, B. navicula 
and others. Its shape (Pl. 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 (Pl. XXIII, fig. 6, @ and ¢) 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 (Pl. XXIII, fig. 6, 6 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 nearly 
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. — 
Palpi 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 (Pl. XXIII, fig. 8) has five teeth; teeth 2, 3, and 4 are double. Tooth 3 
has the larger one broad and flat. Distance between extremities of teeth t and 2, one and 
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SIBOGA-EXPEDITIE XXXI6. 29 
