28.2 BALANIDiE. 



flatter, with the walls strongly ribbed up to the orifice, which is more 

 rhomboidal: the basis is much more cup-formed and more deeply im- 

 bedded in the coral ; but these differences by themselves are by no 

 means sufficiently diagnostic. 



Scutum : the lines of growth are crenated, causing the surface to be 

 very obscurely striated longitudinally : the articular ridge is very 

 prominent, as can be best seen from the outside, and runs down the 

 whole length of the tergal margin with a very regular curve, and hence 

 differs from the articular ridge in the foregoing species. The adductor 

 ridge is either absent, or very indistinct, and parallel to the articular 

 ridge : there is a deep little pit for the lateral depressor muscle. 

 Tergum (7 d)> with the apex somewhat produced or beaked, and tinged 

 purple : external surface almost flat, without any longitudinal furrow : 

 scutal margin curved. Spur very short, placed quite close to the basi- 

 scutal angle of the valve ; broad as half the valve ; lower end square. 

 Internally, the articular ridge is prominent only in the uppermost 

 part of the valve : crests for the depressores very feeble. 



Parietes : their internal surface is very strongly ribbed longitudinally, 

 the ribs being coarsely denticulated at their bases, and finely fluted along 

 their sides. The sheath is transversely ribbed, and clothed with an 

 epidermis furnished with transverse rows of fine hairs. The radii are 

 of a dead white, whereas the parietes are translucent ; the summits 

 are parallel to the basis ; they are broad ; the radii of the carino-la- 

 teral compartments appear extraordinarily broad, owing to the narrow- 

 ness of the parietal portion : the sutural edges are furnished with 

 coarse septa, which are sinuous, irregular, and obtusely denticulated ; 

 the interspaces are filled up solidly. The alee are thin, with their 

 sutural edges almost smooth, and their summits oblique : in some 

 specimens, during the diametric growth, a mere, almost thread-like 

 ribbon is added to their sutural edges. Basis slightly cup or saucer- 

 shaped ; moderately thick, permeated by fine pores, and generally 

 ribbed in lines radiating from the centre. The walls and basis adhere 

 together very firmly. 



Mouth : labrum with six teeth: mandibles with five teeth; the three 

 upper teeth being sharp, narrow, and unusually prominent ; the two 

 lower teeth minute and sharp ; maxillae without a notch. Cirri much 

 injured : first pair with one ramus apparently one third longer than 

 the other : segments not very protuberant : the posterior cirri have 

 elongated segments with five pairs of spines. 



Affinities. — This species is very distinct from all the foregoing : in 

 the carino- lateral compartments being so narrow, and tending, as we 

 may suppose, to become aborted ; in the form and structure of the 

 whole shell, and in its habits, this species shows an affinity and passage 

 to the coral-inhabiting genus Creusia, which has only four compart- 

 ments. There is also a close affinity to the sub-genus Acasta. This 

 species is so closely allied to the following, that I at one time felt some 

 doubts whether they ought to have been specifically separated : it is 

 also probably closely allied to B. terebratus, but the materials hardly 

 suffice for judgment : it is also related, though less obviously, to 

 B. vestitus. 



