BY K. M. JOHNSTON, F.L.S. 

 DIVISIONS OF THE SHELL. 



II. Apex, the posterior end of shell, or 



nucleus 



1. The nucleus may be reversed or 

 sinistral, turning to the left as in 

 the body whorls of Physa 



2. Dextral, turning to the right, normal 



condition 



3. Oblique, when the nuclear whorls 

 are set at an angle, unconformable 

 with the body whorls 



III. Whorl, a single complete revolution of 



the spiral cone ... 



1. Periphery, that part marking its 

 greatest circumference 



2. Suture, the line of channel formed 

 by the junction of the whorls 



3. Body-whorl usually capacious, the 



last turn ending with the aperture 



IV. Base, anterior extremity opposite end 



to apex, usually the front of the 

 aperture 



1. Oblique, as in 



2. Concave, as in 



3. Convex, as in 



4. Plat, as in 



V. Aperture, the open mouth of the shell, 



the interior of which may be simple 

 or entire, or may be channeled by a 

 gutter or canal, more or less pro- 

 duced or everted (2.3). The aperture 

 is : — • 



1. Longitudinal, when its greatest 



diameter is parallel to the axis of 

 whorls 



2. Transverse, reverse of longitudinal 



3. Oblique, greatest diameter oblique 



to the axis 



4. Rounded, the circle slightly in- 



terrupted 



5. Auriform, ear-shaped ... 



6. Ovate, egg-shaped 



7. Oblong, rounded above and below, 

 longer than wide 



8. Lunate, semi-lunar, semi-circular... 



9. Triangular 



119 



Example fig. 

 No. 



IP 



8 a 21 a 



5 C 



36* 



IP 

 20 

 20 

 39 

 11-12 



27 

 13 



37 



9 

 35 

 36 



17 



16 



