VARIATION IN SCULPTURE. 
7.3 
when the interference of the larval shell is removed. Other species 
also possess peculiarities of a more or less striking and distinctive 
character ; but all these youthful characteristics cease to be produced 
after the youthful stage of growth is passed. There are likewise 
developed in the youthful stage of Pisidiam kensloivanum peculiar 
eave-like or wing-like projections, which are naturally formed at the 
margins of the valves in the young shell, but as growth proceeds the 
position they occupy is relatively and gradually altered thereby until 
at maturity they are found at the umbones (see page 42, figs. 103, 104). 
As previously remarked, spiral peculiarities in the Univalves, and 
radiate ones in the Bivalves, arise from the effect of the continued 
action during growth of those parts of the mantle forming the sculp- 
ture, which is in fact moulded upon its surface; the transverse or 
isolated sculpture being the outcome or result of seasonal or periodic 
action of the secretory glands. 
Abrasion has been held to sufficiently account for the smoothness of 
shells usually distinctly sculptured, and although friction may account 
for the smooth and 
polished state in 
marine shells, it is 
not always the true 
explanation. Speci- 
mens or colonies of 
Clausilia bidentata 
are sometimes met 
with ciuite smooth, 
and this smoothness 
is evidently not in- 
variably caused by attrition as suggested by Dr. .Jeffreys, but is 
owing to arrested development, the sculpture in them having never 
existed, or at any rate not proceeded to completion, and indicating 
non-possession and not loss. This partial exercise of qualities is seen 
in many other ways amongst mollusks. 
Fig. 162 . — Clausilia bidentata 
\ 2 lX . la Turton X 2, 
Birmingham, 
Collected by Mr. J. Hopkins, 
Showing the smooth surface 
of the whorls arising from the 
arrested development of the 
regular sculpture. 
IK;. 163. — C, bidentata var. 
septentrionalis A. Schmidt x 2, 
Gairloch, Ross-shire, 
Collected by Mr. A. Somerville, 
B.Sc., F.L.S., 
Showing the usual sculpturing 
of the whorls characteristic of 
the species. 
Hairy processes of the periostraca, which may be differentiated 
from spinous processes by being destitute of any interior calcareous 
support or base and therefore entirely composed of chitinous 
matter, are a characteristic of many species living in moist shady 
places and hiding during the day beneath stones, under decaying 
