16 NERITID^. 



Animal Mantle closed, except a passage for the foot and two 

 syphons ; retracting muscular cords united into a single 

 bundle with one point of attachment. 



D. POLYMORPHA. Pallas. PL III, fig. 3 and 4. 



The remarkable rapidity with which this shell has 

 spread in numerous localities on our island, is one of 

 the most interesting facts connected with it, showing 

 how readily species become naturalized when the 

 climate and situation are suitable. Naturalists are un- 

 decided how it has got introduced, whether adhering 

 to the bottom of ships it has survived a passage through 

 the salt water of the sea from the mouths of rivers in 

 Sweden, Russia, &c.; or by adhering to the timber ex- 

 ported from those countries, it has remained alive for so 

 long a time out of water. 



Hob. Most of our canals which communicate with 

 seaport towns in the midland and northern counties, 

 living in gregarious masses on stones and pieces of 

 timber. 



NERITIDJS. 



NERETINA. Lamark. 



Shell Semiglobular; without an umbilicus ; generally thiclc ; 

 spire of few whorls, last whorl large; aperture semi- 

 circular, furnished with an operculum having a tooth on 

 its lower margin. 



Animal Tentacles awl shaped ; eyes on short pedicels at the 

 outer side of their base ; foot rather short, triangularly 

 oblong. 



N. FLITVIATILIS. 'Linnaeus. PL IV, fig. 1 and PL 

 VIII, fig. 1. 



The peculiar markings of this shell, together with its 

 form, will readily distinguish it from any other of our 

 species. The dark ground is variously chequered with 

 spots of yellowish white, brown, purple, or pink, but 

 they are often obscured by an incrustation from the 

 water, which seems to serve as a protection from animals 

 which would otherwise seize them as prey. 



