76 
HELICIBiE. 
They have generally been united with the Arions ; 
but it has lately been discovered that there are 
animals with well-developed shells that agree with 
the Arions in character^ as the Helices do with the 
Slugs ; and it has therefore been thought better to 
separate them by the above character into two 
groups, rather than follow Lamarck in dividing these 
animals into groups, by the gradually and greatly 
varying form of the body. 
The shells are covered with a distinct periostracum. 
It has generally been believed that shells which are 
covered with a reflexed portion, or imbedded in the 
mantle, are destitute of this covering ; and they 
have been separated from other shells for this reason. 
But this is an error arising from the theory that the 
'periostracum of shells is analogous to the scarf-skin of 
vertebrated animals, instead of its being merely the 
part, consisting almost entirely of animal matter, 
that is first deposited by the animal when it is about 
to enlarge its shell, and which forms the basis of the 
new part of the shell, afterwards strengthened and 
thickened by the addition of the chalky matter 
within it. 
These animals sometimes suspend themselves by 
a kind of thread formed from the viscid secretion 
which covers their body ; hence one of the smaller 
ones has been called Limax Jilans. The eggs are 
covered with a transparent coat, and are often 
united together by a membrane like a string of beads. 
They are laid between May and September. They 
are hatched in about 25 or 30 days, and the young 
reach their full size near the end of the year. Limax 
