310 BALANID.E. 



Scuta, of the usual Bab-triangular shape, and not transversely elon- 

 gated, as in T. purpurascens. External surface striated longitudinally; 

 in many specimens there is a medial depression, or a row of very small 

 pits, such as occur on the scuta of Balanus trigonus and Icevis. The 

 adductor ridge is moderately developed, and runs nearly parallel to 

 the occludeut margin; there are no crests for the rostral and lateral 

 depressor muscles. 



Terga : these in area equal two thirds of the scuta : the spur is short 

 and rounded, and placed as described under T. purpurascens ; but the 

 articular ridge seems to be more prominent than in that species. 



Structure of the Shell and Radii. — The parietal tubes are small, and 

 very numerous, as in T. purpurascens. The radii are wide, square on 

 the summits, but not so conspicuously covered by hirsute epidermis as 

 in that species. Internally, the tubes forming the radii are smaller, 

 and run more transversely than in T. purpurascens, that is in the normal 

 course, as in Balanus. The edges of the alae are nearly or quite 

 smooth. The Basis is as distinctly calcareous, as in the other species 

 of the genus. 



Mouth: the tropin are not so hairy as in T. purpurascens; the 

 labium seems destitute of teeth ; the palpi are club-shaped at their 

 ends ; the mandibles have only three teeth. The second and third 

 cirri are not so short and blunt relatively to the others as in T. purpu- 

 rascens. In the posterior cirri, the elongated segments carry four main 

 pairs of spines, between which there is no intermediate tuft of fine 

 spines. 



The Affinities of this species have been fully pointed out under the 

 last and closely related species. In external appearance, T. costata can 

 at first hardly be distinguished from those young and pale-coloured 

 varieties of T. purpurascens, which have their external surface not cor- 

 roded, and their radii well developed. 



6. Tetraclita vitiata. PL 11, fig. 3 a — 3 e. 



Shell white, generally tinged in the tipper part with pink ; 

 surface irregular : parietal tubes very irregular : radii 

 moderately wide, with their summits oblique : alee with very 

 thick crenated sutured edges : tergum with the spur not joined 

 to the basi-scutal angle ; spur with its extremity equably 

 rounded. 



Hab. — Philippine Archipelago ; Barrier Reef, (Raine's Islet), Australia ; 

 Mus. Cuming and Stutchbury. Attached to massive corals, to coral-rock, to a 

 Tridacna, and to Tetraclita ccerulescens. 



General Appearance. — Shell conical, moderately steep : white, gene- 

 rally with a tinge of pinkish-purple in the upper part, owing to the 



