THE STUDY OF SHELLS. 



149 



with a long turret, and such a shell as the cowrie, a short 

 turret, both with a similar number of turns in the spiral 

 (Fig. 53) . How is the difference explained ? If we break 

 a cowrie shell, the greater part of 

 the earlier turns of the spiral will 

 be found overlapped by the later 

 ones, and the last turn covers all 

 except the tips of all the rest. In 

 old shells the spiral may be ob- 

 literated on the outside, but of 

 course it persists within. 



As far as spiral shells are avail- 

 able, compare as to the number 

 of turns and amount of overlap- 

 ping. Draw a series of types. 



Some special cases of interest 

 are the ram's horn shell (Planor- 

 bis) found in fresh waters, whose 

 spiral is flat, not turreted; the 

 ear shell or "oriner" (Haliotis), 

 whose spiral is so open that there 

 is 110 pillar, and which is greatly 

 drawn out to one side ; the limpet 

 (Patella) cap-like, whose spiral is 

 present only in the youngest stages. The fresh-water 

 limpet (Ancylus) is a small helmet-like form occurring in 

 ditches, etc., which may be compared with this last. 



Fig. 53. Cowrie shell broken 

 to show the parts o? the 

 spiral concealed within. 



