272 THE VOYAGE OF H.M.S. CHALLENGER. 



Shell. — Very small, strong, white, rounded, narrowed in front, broad and gibbous 

 behind, where also the labral callus, somewhat pinched out, and prominent especially on 

 the right, encircles the top of the mouth, which is much bent and narrow, channelled in 

 front and behind, and runs the whole length of the shell ; the outer lip is varixed and 

 inside is finely toothed, the inner lip is bent, unthickened, and has three very feeble teeth 

 in front. Sculpture: none, but some very faint lines of growth. Colour translucent 

 white. Spire entirely concealed. Moutli is small, much bent, the full length of the 

 shell, a very little expanded behind, very slightly narrowed in the middle, but nearly of 

 the same breadth throughout. Outer lip thick, curved, but less so above than in front, 

 where it is oblique in direction ; in the middle it is slightly incurved and is blunt, but 

 above and below is bevelled outwards to a sharp edge, the inner side of which is furrowed 

 with small fine teeth ; it is thickened externally by a narrowish, slightly prominent labral 

 varix, which is continued, a little weakened but with a small exterior furrow, round the 

 outside of the posterior canal, which is deep and rounded, and truncates the tip of the 

 shell. Inner Up very much bent, particularly above ; at the top it is shortly, thickly, and 

 angularly bordered by the end of the callus of the outer lip, which coils round the top of 

 the shell ; in front there is an almost imperceptible thickening where the teeth occur ; 

 these are four in number and very small, the upper two being barely visible ; the lower 

 two are very oblique, and the under one of these forms a minute flange to the point of 

 the pillar, which is very truncated. H. 0'082 in. B. 0-055. Mouth, breadth 0-007, 

 greatest breadth (at the top) 0-012. 



This little species resembles, of course, a considerable number of small Marginellas, of which 

 there is a large group, only distinguishable from one another by careful examination of specimens. 

 So obvious a fact as this need not have been stated, but for the almost incredible tendency to mix up 

 a number of these species on the mere testimony of figures and descriptions. Marginalia agger 

 differs from Marginclla guanclia, d'Orb., in that the shell is rounder, the tumidity of the body is 

 less gibbous, the mouth is more prolonged behind, and the tip of the shell in front is less truncated, 

 the teeth on the outer lip are a little stronger, and those on the pillar a little weaker than in that 

 species. Compared to Marginclla clandestina, Brocchi, Marginclla agger is rounder in form, being 

 less broad behind, but is more pointed at both ends, behind especially ; the presence of the little 

 projections formed by the labral varix is a marked feature ; there, too, the canal is deeper, the pillar 

 teeth, too, are much finer and feebler. From Marginclla occulta, Monterosato, it differs markedly in 

 form, being smaller, shorter, and rounder, the posterior enlargement of the labral varix is much 

 more prominent, and the posterior oral canal is much deeper ; it lacks, too, the slight basal excava- 

 tion which is always found in that species. Marginclla ovuliformis, d'Orb., from Cuba, has a much 

 shorter mouth, and lacks the posterior projection. 



