78 
VARIATION IN THICKNESS. 
The opposite oi’ incrassate condition may be well illustrated by 
TIf'U.v a.'^persa var. solidissitna Paulucci, a specimen of which, found 
by the late j\Ir. C. Ashford on the rurbeck limestone at Swanage, 
though of oidy normal size, weighed eighty - three grains; and 
Mr. C. Jefferys has taken several specimens on the cliffs at Tenby 
weighing over one hundred grains each, or more than three times 
the weight of average individuals. 
fh'li.r uemoralis normally weighs about 1 1 grains, yet a thick shelled 
race, the variety crctkola of Miirch, which formerlj^ e.xisted in the West 
of Ireland, and in a somewhat less pronounced form has been .shown 
by i\fr. Collier to still e.xist there, attained a much greater weight. 
A number of sub-fossil specimens of this variety, kindly sent me by 
Prof. D’Arcy Thompson, had an average weight of 44 grains, the 
heaviest weighing 78 grains and the lightest, a somewhat diminutive 
shell, weighing la grains. This abnormal incrassation of the shell 
substance is principally caused by the .secretion of additional and 
thicker layers to the inner surface of the shell, and probably arises from 
the mollusk living upon suitable geological strata, furnishing abundant 
supplies of calcic matter, and also from the probable prolongation of 
the life of the animal beyond the usual life-limit of the species, perhaps 
owing to a succession of mild and equable seasons, the periodical de- 
position of the shell matter by the visceral mantle occnrring regularly 
at its proper season during the whole life of the animal. Post-mortem 
deposits of calcic carbonate, by carbonate of lime entering the pores 
in a fluid state, have been suggested as the cau.se of the unusual 
thickness of the.se 
shells, but a care- 
fill e.xami nation of 
sections throngh the 
shell disprove this 
suggestion, as they 
clearly demonstrate 
that this extreme 
incrassation of the 
.shell has taken place 
ill a strictly normal manner, the inner laj^er upon the base of the 
preceding whorls retaining its characteristic comparative teiinity, 
being only modified by the delicate films of calcic matter, deposited 
at the same time as the stronger layers which thicken the interior 
Fir,. 171. — Section through 
the shell of a normal 11. ni'i/io- 
ralis r.., showintj the usual thick- 
ness and other characters of the 
species. Weight of section, 8 
grains. 
From a section cut liy Mr. 
F. Rhodes. 
Fk;. 172. — Section ihroiigli 
the shell of a suh-fosi.il //. ucino- 
ratis F.jfrom Dog’s Hay, Conne- 
mara, e.\hibiting the remarkable 
thickening of the shell-wall. 
Weight of section, 43 grains. 
From a section cut by Mr. J. 
Ray Hardy. 
