02 GASTEROPODA. 



The lingual membranes will be found to be more or less 

 coiled, and usually attached to the jaws. It is desirable, in the 

 mounted specimen, to have the membrane flattened out, with the 

 dentiferous side uppermost, and dissociated from the jaw. Some 

 species have a large, strong jaw, which, if left with the lingual 

 membrane, will raise the cover-glass so far above the denticles as 

 to exclude the use of the higher powers of the microscope. 

 Therefore, some mechanical work is necessary to unfold the 

 radula, and remove the jaw. Having provided a clean glass slide 

 on the turn-table, the specimen is taken from the clove oil and 

 centered on the slide. Now placed under the microscope pro- 

 vided with an erector, and using mounted needles, the radula is 

 easily unrolled with the dentiferous side uppermost and the jaw 

 removed. Replaced upon the turn-table, a thin cover-glass is 

 superimposed and centered. The cover-glass should be put on 

 before the balsam is added, as it prevents the specimen from again 

 becoming coiled or displaced. A drop of balsam in benzole is 

 put adjacent to the edge of the cover, and the slide held an 

 instant over a gas-burner or alcohol-lamp, which will cause the 

 balsam to flow by capillarity under the cover-glass. A small 

 spring-clip is then used to press the cover down and hold it in 

 place. The slide is removed to a drying oven, and left until the 

 balsam has hardened, so that the portion outside the cover can be 

 scraped off. The slide is then cleaned by washing in strong 

 alcohol, using a piece of soft tissue paper to rub it dry. It is 

 quite essential to use cover-glasses of known thickness. Many 

 radulae require a one-tenth inch objective. The convexity of the 

 object, combined with the thickness of the cover, necessitates tiie 

 use of very thin glass. For the Rissoidc^, I have usually em- 

 ployed glass of "004 inch thickness. 



Movement of the Earth's Surface.— At the equator, a 

 point on the earth's surface moves rather more than 1,000 miles 

 an hour; in latitude 45 deg., north or south, the rate of motion is 

 about 750 miles an hour. London is carried round the earth's 

 axis at the rate of more than ten miles per minute. 



