12 HiCKLING, Variation of Planorbis Diultiformis, Bronn. 



variations in shouldering and channelling, but also by the 

 degree of development of the dorsal carina. 



(4) Umbiliais. In the early stages of growth, the 

 .-v'idth of the umbilicus varies greatly, according to the 

 length of time during which any individual maintains a 

 more or less discoid habit. The differences which arise 

 from this cause may be readily appreciated by a compari- 

 son of Figs. 4 and 5. In the adult shells there is likewise 

 a general tendency for the umbilicus to become more 

 narrow among shells with a high spire than among those 

 of a " lower " form ; but, in addition to this variation with 

 height, the umbilical width in the mature shells varies 

 considerably as a result of the varying form of the base 

 of the whorl. This latter variation, among shells of the 

 same height, is shown in PL /, Fig. 2 A-D, though these 

 shells were not selected for this purpose. It is evident 

 that all the variations in umbilical width are continuous 

 in character. 



(5) Aperture. The form of the cross-section of the 

 whorl is affected by all the variations already noticed, and, 

 in addition, the actual aperture of a completely mature 

 shell may present further independent modification. The 

 changes in apertural form with height are seen in PL I., 

 P^^S- 7- The flattest adult shells always have the 

 whorls strongly quadrangular in section, corresponding 

 with a strong development of both dorsal and ventral 

 carinje. As the spire becomes higher, the aperture tends 

 to become pear-shaped, the change in form being perfectly 

 gradual. Carination always affects the form of the 

 aperture, producing more or less angularity in its outline. 

 When a shoulder and channelled suture is developed, the 

 aperture shows a well-marked superior sinus correspond- 

 ing to the shoulder. Finally, the base of the whorl varies 

 considerably as regards the degree of flattening or round- 



