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ribs coloured more distinctly and transversely dotted with darker red. Radii rather broad, not 

 very distinct, however, in the specimens with corroded surface; coloured white or faintly reddish 

 in the specimens with ribbed surface, and being transversely striated in these specimens, more 

 distinctly so in the upper half. Orifice small, elongately pentagonal, nearly entire, certainly 

 not toothed; the recurved tips of the terga occupy the carinal angle of the orifice in a very 

 peculiar way. Together with the extreme narrowness of the carino-lateral compartments, 

 causing the radii of these valves and those of the rostro-lateral compartments nearly to touch 

 each another, the characteristic appearance of the tips of the terga will always make it easy to 

 recognize this species. 



Parietes (PI. XXI, fig. 7, a, 6 and c) solid, no pores, but with well-developed longitu- 

 dinal ribs, forming so many septa, which are denticulate in their basal parts. The sheath is 

 transversely ribbed. The radii of the carino-lateral and rostro-lateral compartments nearly touch 

 each another, owing to the extreme narrowness of the carino-lateral compartment (PI. XXI, 

 fig. 5). The sutural edges are coarsely ribbed transversely. The basis is thin, attached firmly 

 to the surface of the calcareous plate and has no pores ; it adheres very firmly to the walls 

 of the valve and is perfectly flat — not cup-shaped, and not imbedded in the coral at all. lts 

 interior surface shows delicate riders radiating from the centre. 



Greatest diameter of the basis of one of the larger specimens about 7 mm. 



S cu turn (PI. XXI, fig. 6, a and c) the lines of growth are very distinct and are beset 

 with a number of small points making them look as if crenated; occludent margin distinctly and 

 more or less irregularly toothed; articular ridge not very prominent and extending down from 

 the apex to about two thirds of the length of the tergal margin ; adductor ridge indistinct ; 

 cavity for the lateral depressor muscle well-developed ; along the tergal margin the surface of 

 the valve is uplifted, the growth-ridges being here very distinctly crenated or toothed. 



Tergum (PI. XXI, fig. 6, b and d): apex distinctly produced, beaked, carinal margin 

 convex, scutal margin strongly hollowed out, basal margin sinuous. Ridges of growth not very 

 distinct, external surface almost flat, without a longitudinal furrow; spur situated quite close to 

 the scutal angle of the valve, its extremity being almost confluent with that angle, not very 

 short, and not quite half as broad as the valve itself. Crests for the depressor muscles indicated, 

 but not very prominent. Articular ridge distinct only in the uppermost part of the valve. 

 Tergum extremely brittle — it is difficult to prepare a really faultless specimen of it. 



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



Mouth. Labrum (PI. XXI, fig. 8): The outlines of the lateral parts of the labrum 

 together make an angle; the notch is deep and not very wide at the entrance. Three 

 well-developed and sharp teeth on each side of notch. 



Pal pi elongate, nearly oval, not club-shaped. Hairs along upper margin form a dense 

 and regular row. Those on outer surface long, and placed in rows which make angles with 

 the inferior margin near distal extremity. On the inner surface a longitudinal series of shorter 

 hairs is disposed and this runs parallel to the upper margin, the hairs overlapping the edge 

 of the labrum when the palpus is directed downwards. Extremely delicate ciliae are disposed 

 along part of inferior margin and rounded distal margin. 



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