110 THE VOYAGE OF H.M.S. CHALLENGER 



This species is so thin and transparent that scarcely any trace of nacre can be recognised ; but 

 by carefully occluding the light on the outside, and thus having all the light reflected from the inside 

 of the mouth, the pearly lustre is distinctly visible, especially at the outer upper corner. 



4. Seguenzia trispinosa, 1 Watson (PI. VII. fig. 4). 



Seguenzia trispinosa, Watson, Prelim. Keport, pt. 3, Journ. Linn. Soc. Lond., vol. xiv. p. 591. 



Station 120. September 9, 1873. Lat. 8° 37' S., long. 34° 28' W. Pernambuco. 

 675 fathoms. Pied mud. 



Sliell. — Small, high, conical, scalar, with three rows of tubercled lirations, umbilicate, 

 thin, smooth. Sculpture: There is a sharp circumbasal carina, above this is a broad 

 shallow furrow ; about one-third up the whorl is a narrow and blunt liration ; a little 

 more remote is a third, separated by a narrow, horizontal, flat surface from the suture. 

 All these three lirations are ornamented with little tubercles or blunt spines, which are 

 strongest on the highest thread, and there number about twenty-five on the body-whorl, 

 while on the second thread they are about twenty-eight. The base, which is rather flatly 

 arched, has round the outside a flat surface hardly deep enough to be called a furrow, 

 defined on the inner side by a clear narrow line, within which the curve of the base rises 

 a little and has some faint spirals. The edge of the umbilicus is sharply defined by a fine 

 line, outside of which is a broad shallow furrow bordered externally by a slight spiral ; 

 then comes another narrower furrow, the outer side of which is the most projecting part of 

 the base ; beyond this the base is rather flat and has some obsolete spirals. On the upper 

 whorls the spirals are feeble and without tubercles, which only appear distinctly on the 

 fourth whorl. Longitudinals — the flexuous lines of growth are very faint. Colour por- 

 cellanous when young and fresh, but weathering to a chalky white, with a pearly nacre 

 below the thin surface and within the mouth, especially at the outer upper corner. Spire 

 conical, high, scalar. Apex very small and sharp, flattened on the one side, and with the 

 minute 1^ embryonic whorl projecting tumidly on the other. Whorls 7\, of gradual 

 increase : the upper ones are rounded ; the latter flat below the suture, then angulated, 

 then flat on the conical slope of the spire, and then a very little constricted above the carina ; 

 on the base they are very slightly rounded, with a flat and feebly impressed but sloping 

 border round the outside ; they are sharply angulated at the umbilicus. Suture linear, but 

 strongly defined by the constriction and impressed angulation of the shell at that point. 

 Mouth perpendicular, nearly square. Outer lip sharp and thin, not patulous, not descend- 

 ing. The curves are very faintly indicated by the lines of growth, but are similar to those 

 described in Seguenzia monocingulata, Seg., there being three sinuses, one near the suture 

 between the first and second spinose threads, a second, very small but sharp, at the carina, 



1 From the three rows of spinous lirations. 



