372 Miss B. Lindsay on the 



in wliicli the ridges usually alternate with those of the 

 former. 



(4) After this, those rotatory movements begin whicii 

 have been so often described. So far as I have observed, 

 the earlier writers are correct in stating that the rotatory 

 movements are in the direction of the movement of the hands 

 of a Avatcli, i. e. from right to left, the animal biing in its 

 usual position, siphons up and pedal end down. 



The shell-scraping activity of the mollusk was, on one 

 occasion, made audible — when, the wall of a burrow in shale 

 having been partly destroyed, the animal came into contact 

 with the concrete floor of the tank. The sound continued for 

 hours, on several successive days, and much resembled the 

 gnawing of a mouse in wood. Probably the rotatory siiell- 

 scraping movements might be rendered audible at any time, 

 by the use of the microphone. 



The boring powers of Zirpluea are materially assisted by 



The Chemical Composition of the Shell. 



The earlier observers established the fact that these shells 

 consist of aragonite, which, being harder than the usual 

 calcite, better enables them to bore. This alone should 

 almost have sufficed to establish the truth of the mechanical 

 theory of boring. 



In recent years Meigen's wet test (cobaltic nitrate) has 

 afforded a convenient means of confirming the evidence given 

 by optical appearances regarding the composition of the 

 shell. 



In view of the fact that in some mollusks the shell has 

 been described as. consisting of alternate layers of aragonite 

 find calcite, it is interestinii- to note the deceptive ajipearances 

 presented by the slu-ll of Zirphiea under both tests. 



(1) Optical, Owing to the fact that the upstanding 

 ridges are set nearly at right angles to the rest of the shell, 

 they appear, in a section examined b}' polarized light, as black 

 stripes, when the plain part of the shell is illuminated. 



(2) In the wet test, the ridges, when viewed externallj-, 

 ])resent streaks of lilac colour instead of the cherry-red 

 coloration characteristic of aragonite. Tiie reason is, that 

 some of the substance of the surface of the ridge passes into 

 solution in the sea-water (or in aqueous spirit, if preserved 

 specimens are used) and is redeposited on the drying shell; 

 it is then aragonite no longer, and gives the calcite test. 

 On breaking the shell across and testing figain, it is seen 

 that tlie ridges are of the same substance as the rest ; it id 



