NASSA. 387 



may present. It is as variable in colour as in shape. It 

 ranges all round the British shores, and extends, in the 

 Euroj^ean seas, as far south as the north-west coast of 

 Spain, though not continuously, so that its presence there 

 is probably only as an outlier. It commenced to appear 

 Avithin our area during the red-crag epoch, and was pro- 

 bably diffused from the American side of the Atlantic. 

 At present it inhabits both sides of the north Atlantic, 

 and ranges through the Icy Seas. 



NASSA. Lamarck. 



Shell usually strong, ovate, rotund or Neritiform, always 

 with a tumid body-whorl, variously sculptured, smooth, 

 ribbed, decussated or striated, rarely with varices : spire 

 acute or obtuse. Aperture ovate, with a short and con- 

 stantly reflected, and as if truncated canal ; outer lip often 

 denticulated within, columellar lip reflected, often expanded 

 and callous, and sometimes toothed. Operculum corneous, 

 unguiculate, nucleus terminal. 



Animal with a lunate not very broad head, beai-ing two 

 long acute tentacula, filiform beyond the eyes, which are 

 placed in the hind jiortions (united ommatophori), extending 

 for about a third of their length ; proboscis long, retractile, 

 with corneous jaws, and a tongue armed with triple rows 

 of teeth, of which the axile one is broad and sublunate, with 

 numerous serrations, the laterals large and hamate. Mantle 

 lax, produced into a long recurved siphon, which extends 

 for a considerable distance beyond the canal of the shell. 

 Foot extensive, expanded, oblong, truncated, and angulated 

 in front, bifurcated at its posterior extremity. Branchial 

 plumes, two. Male organ long, tapering, geniculate, 

 reflected. 



