330 BALANID.E. 



Far. (5) elegans (PL 10, fig. 1 d) : outer lamina preserved, except- 

 ing sometimes near the summit of the shell; white, tinged with yellowish 

 brown from the epidermis ; surface strongly ribbed longitudinally ; 

 orifice rather small; sheath reddish-purple ; terya narrow, with the 

 basal margin sloping as in var. rubescens. 



Far. (6) communis (young) (PI. 10, fig. 1 e) : radii developed, very 

 narrow ; outer lamina of shell preserved, gray or dull purple ; surface 

 slightly ribbed longitudinally . 



Var. (7) patellaris (PI. 10, fig. 1/): radii developed, very narrow, 

 white ; outer lamina of shell preserved, generally reddish-purple; steeply 

 conical, with the orifice extremely small; surface smooth, with longi- 

 tudinal white ribs. Terga very narrow, with the spur sharply pointed, 

 and with the basal margin on the carinal side sloping towards it, or 

 not making an angle with it. Scuta, with the adductor ridge very 

 prominent. Attached to a ship s bottom. 



J£ab. — West Indies, Brazil, West Colombia, Panama, Galapagos Archipelago, 

 California, Philippine Archipelago, China, East coast of Australia, lied Sea ; 

 generally attached to tidal rocks, sometimes to shells, sometimes to wooden 

 posts. Very common. 



General Appearance. — This, the widest-distributed and much the 

 commonest species of the genus, varies greatly in external appearance. 

 The usual shape is steeply conical, but some individuals are much 

 depressed. In the common varieties the outer lamina of shell has 

 been removed even close to the basal edge ; the upfilled parietal tubes 

 being thus exposed (fig. 1 b), as flattened adpressed points. These 

 points are largest in large specimens, but they vary somewhat in size 

 in specimens of equal growth. When the outer surface is preserved, 

 it is generally ribbed longitudinally, but is sometimes quite smooth. 

 The most general colour is dirty gray or dark purple ; but many spe- 

 cimens are pale pinkish-purple, owing to the exposure of the parietal 

 tubes upfilled with shelly matter of this tint : there are also, as given 

 under the characters of the vars., black, white and green varieties. The 

 sheath is always tinted by the prevailing colour. The radii are rarely 

 developed, but generally the four sutures are distinct ; sometimes these 

 are externally quite obliterated, the shell, as seen from the outside, 

 consisting of a single piece like a patella or fissurella. When the radii 

 are developed they are very narrow, with their upper edges oblique : 

 their development seems always coincident with the more or less 

 perfect preservation of the shell, and their function is to enlarge the 

 orifice ; the enlargement being usually effected by the disintegration and 

 removal of the whole upper part of the conical shell. The size of the 

 orifice varies considerably ; in the seventh variety it was extraordinarily 

 small: in outline it varies from oval to rounded trigonal or rhomboids!; 

 in some specimens, with the radii well developed, it was rounded pen- 

 tagonal. 



Size. — Tetraclita porosa is the largest species of the genus ; I have 

 seen specimens attached to a large pebble of granite in the British 

 Museum, which measured two inches in basal diameter, and nearly one 

 inch and a half in height. 



