56 



C. HEDLEY. 



wall of the cone may be strengthened by radial corruga- 

 tions. If a stream be received on one side only, the shell 

 develops a long front slope, and steep rear, but where the 



# pressure is endured on various aspects 

 the apex is central and the shell 

 symmetrical. So in the rapid moun- 

 tain streams of the tropics, Navicella 

 has for the same purpose evolved from 

 Neritina to a limpet like shell. 

 By adaptation to surf environment 



Fig. 18. Catophragmus th& llm P et haS tllUS aiTiVed at the 



poiymerus, & sessile bar- best shape for resistance. A group of 

 naclefrom the surf zone, limpets Helcioniscus variegatus, 1 is 



Showing descent from a T iT-.ix-rrx.-irv i i 



, , o -,. shown by Plate V, fig. 10, as planted 



pedunculate form modi- ° F 



fied to a limpet shape to on a bare space of rock exposed to 

 resist the blows of the the full sweep of the ocean surf at 

 waves - Long Reef. Here there is not the 



least cover or protection of weed or rock. Relying on con- 

 tour alone for their defence, the naked limpets withstand 

 the full force of the waves. 



The shield of the shell is held in position by the limpet's 

 foot. This is a muscular disc closely applied to the rock. 

 If taken unawares a brisk, though slight, tap removes the 

 limpet, but if warned, the limpet can resist a pressure 

 which Reaumur measured as thirty pounds weight. The 

 exact operation of the mechanism is not ascertained, 2 but 

 it is probably concerned with atmospheric pressure, or in 

 popular language, suction. 



Other molluscs, whose shells are less perfectly moulded 

 for evasion, rely for defence on the strength of some detail 

 of construction. Characteristic of rocky surf-swept head- 



i Hedley, Proc. Linn. Soc. N.S. Wales, xxxix, 1915, p. 714. 

 8 Davis and Fleure, Liverpool Marine Biol. Committee, Memoir x, 

 Patella, 1903, p. 4. 



