Heart-Cockle and Fresh-water Cockle 109 



Lake Orb-sliell (>S'. lacustre), ^vhen looked at from the 

 side is much like S. corneiim, but from either end it 

 is seen to be somewhat compressed, the beaks central, 

 and usually capped by the nucleus of the shell ; very 

 thin, semi-transparent, and glossy, of a 

 greyish-brown hue. The white animal has 

 a foot twice the length of its shell. The 

 siphons of this mud-loving species are long, 

 and exhibit a marked difference between 

 the respiratory and excretory tubes : the former being 

 cylindrical with a wide mouth, whilst the latter is 

 slightly conical with a small mouth. It is well 

 distributed over England and Wales in still waters, 

 but is only foiind locally. 



As already indicated, the five species of Pea-shells 

 (Pisidium) may be recognised as such at a glance by 

 their possession of one siphon instead of two. Their 

 shells are also much smaller and less equal-sided than 

 those of the Orb-shells. The River Pea-shell (P. 

 amniicimi) is solid, glossy, somewhat tri- 

 angular, and swollen ; deeply grooved, 

 greyish brown in colour, with a thick epi- 

 dermis, and a quarter of an inch in length. 

 The greyish animal is almost transparent, 

 with a long broad foot and a short siphon. It is 

 generally distributed in running waters. The Foun- 

 tain Pea-shell (P. fontinale) is similar in shape to the 

 last, but smaller, less solid, faintly grooved, of a paler 

 grey colour, with more prominent beaks, and a thin 

 epidermis. It is widely distributed in similar situa- 

 tions to those affected by the last named. The Small 

 Pea-shell (P. pusillwun) is oval and thin, with little 

 gloss, finely but irregularly grooved ; of a brow^nish- 



