54 NERITID^. 



and almost concave below, solid, moderately glossy, yellowish 

 or brown, with often brown or white zigzag streaks, spots, or 

 bands, which run lengthwise or in a spiral direction, and 

 marked with fine but distinct transverse striae or plaits, which 

 are more conspicuous towards the suture : epidermis thin : 

 whorls 3, rather convex, the last or lowermost exceeding two- 

 thirds of the whole shell, increasing very rapidly and dispro- 

 portionately in size : sjnre very short and oblique : suture 

 rather deep : mouth, or aperture, semilunar : outer lip sharp : 

 pillar-lip exceedingly broad, polished and flat, with a sharp 

 and plain edge : operculum semilunar, glossy, of an orange or 

 yellowish colour, marked spirally with two or three slight 

 grooves, and ti'ansversely with numerous and flexuous striae ; 

 its external edge is thin and has a border (sometimes two) of 

 black or yellowish-red, both above and below ; its internal 

 edge is thick ; the spire of the operculum is placed at the 

 lower side, and formed of 1^ or 2 whorls; attached to the 

 under side of the opercular spire is a singular jirocess, re- 

 sembling a second but much smaller operculum, which has 

 also a thickened edge on the inside, and projects obliquely so 

 as to act as a bolt in fastening the operculum to the pillar-lip. 

 L. 0-35. B. 0-25. 



Habitat : Slow rivers^ streams, and lakes through or 

 into which water flows, or having a stony or gravelly 

 bed, in all parts of the kingdom, fi'om the Orkneys to 

 Cornwall. It has been found in the peat-bed at New- 

 bury ; but this is not, I believe, recognized as belonging 

 to the upper tertiary formation. This species ranges 

 from Finmark to Algeria and Sicily, where the form or 

 variety called by Lamarck N. Bcetica prevails. A dwarf 

 variety has been described by Nilsson, which inhabits the 

 shores of 'the Baltic Sea, adhering to seaweeds and 

 stones, sometimes at a distance from the mouth of any 

 river, and living in company with the common Mussel 

 and a few other decidedly marine shells. He also noticed 

 that these last are similarly dwarf forms. A variety in 

 which the shell is quite black has been found by Mr. 

 North in the Ouse. Many other varieties have been 



