THE MOLLUSCA 



969 



places, as the teeth are beautifully shown and often stand detached 

 on the edges of the cut. 



"For preservation for future study, the glycerin preparations 

 sold by the opticians will be found useful, though they have the 

 great disadvantages of deliquescing in warm weather." 1 



The radula may also be mounted in Canada balsam. In this 

 case it should be stained with carmine or chromic acid, as otherwise 

 the specimens will in time become transparent. 



MEASUREMENT AND DESCRIPTIVE TERMS 



The length or height of a univalve shell is the distance from the 

 apex to the basal edge of the lip, measured along a line drawn 

 through the axis. 



The greater diameter is the greatest width, including the lip, 

 measured on a line drawn at right angles to the axis. 



The lesser diameter is measured on the same plane, but on a line 

 at right angles to the greater diameter. 



Shells are dextral or sinistral, accordingly as the aperture is on 

 the right or left of the axis, when 

 held, apex uppermost, with the 

 aperture facing the observer. 



In bivalve shells, the length is 

 the distance from the anterior 

 to the posterior extremity; the 

 height is the distance at right 

 angles between two parallel lines 

 so drawn as to touch the highest 

 dorsal and lowest basal points; 

 the width is the greatest diameter 

 measured in a line at right angles 

 to the basal line. 



--5 



FIG. 1403. The shell of a univalve. 



The remainder of the terms in 

 common use are sufficiently indi- 3 

 cated on the following diagrams.- 



1 W. G. Binney, Man. Am. Land Shells, p. 44. For full instructions in regard to 

 the preparation of the radulae of the minute species, see Beecher, Journal N Y. Micro- 

 scopical Society, 1888, p. 7. 



Apex. 6. Umbilicus. 



Spire. 8. Suture. 



Operculum. 10. Columella. 



Lip. 1-5. Height. 



7 -9. Greatest diameter. 



