Key to Plate I — Continued 



Fig. 18. Shell 40 x 38. 



Fig. 19. Shell 45 \ 42. Velum and tip of foot protruded. 



Fig. 20. Shell 50 x 40. Foot protruded. 



Fig. 21. Shell 55 x 48. Full size of the larva. Sizes larger than 

 this are seldom taken in the plankton net, showing that at tins stage mosl 

 larvae either become attached or are destroyed. 



55 x 6-9=379-5//= -3795 mm.= l/66 inch. 



Fig. 22. Spat of a few hours' fixation, with a narrow rim of spal shell 

 (dissoconch) built on to the lower border of the larval shell (prodissocouch) . 

 This stage is difficult to find on natural objects (stones, shells, etc.), but 

 may best be obtained on clean surfaces such as glass intentionally put 

 out to capture them about the middle of August and later. 



