VARIATION IN BANDING. 
99 
Fig. 20F — I/dix nenioraiis L. 
formula 
Spurn Point, Yorkshire, 
a specimen as the one fi^urerl (fig. 20.S), wliicli possesses a slender 
bandlet eipiidistant between tlie tliinl and fourth hands, would be 
clearly indicated by the formula 12.9x b'5. 
Indistinct, irregularly developed or spotted bands are indicated by 
a colon in place of the numeral — an in- 
distinct third band, provided the remain- 
ing four bands are normally developed, 
would be shown by the formula 12:4.5; 
whereas if all the bands are indefinite 
or irregular, the peculiarity would be 
11,1 P .1 p 1 Collected bv Mr. W. E. Clarke. 
expressed by the use oi tlie lormula f.l.s. 
as in fig. 204. Some authors, however, consider that any of the 
normal bands when indistinct or rudimentary are more suitably 
indicated by the use of the small numeral, which I recommend 
.should be restricted to represent the split-ofi’ bandlets. 
A .specimen of HeU.r hortensis from Folkestone, kindly given me by 
Mrs. Fitzgerald, appears to have eight .slender but distinct bands, on 
account of their colouring matter being mainly concentrated at the 
edges of each band, leaving the centre of 
each band very little, and in some portions 
not at all darker than the general ground 
colour, this separation of the margin of 
the bands gradually becoming more dis- 
tinctly marked as growth proceeds, and 
furnishing a very instructive illustration as to how the multiplication 
of bands may come about, this specimen would be indicated by the 
formula ] 2 m (fig. 20,5). 
This division of, or breaking up of the banding is a modification 
which has been ascribed to the effects of aridity or dryness, either of 
season or locality, and that this cause probably induces band disin- 
tegration, is shown by the fact that Mr. W. E. Clarke collected for 
me, at Whitsuntide, 1<S82, several thousands of Thdix nemmiUh from 
the .sand-hills at Spurn Point, Yorkshire, a locality which, according 
to the meteorological reports, is one of the driest spots in the kingdom, 
and it is remarkable that not a dozen of the immense number 
gathered e.xhibited evenly developed and strongly marked banding, 
the bands when present were all more or less broken up and discon- 
nected (see fig. 204) and this circumstance furnishes modified 
evidence of the evolution of bandless species in desert districts; this 
Fig. 205. — Helix kortetisis Mull., 
formula 12.4.'i4455, 
Folkestone, Kent, 
Collected by Mrs. Fitzgerald. 
