THE SHELL IN UNIVALVES. 
23 
has demoustrated that a 10 per cent., or even a weaker, solution of 
hydrochloric acid gives the most perfect and satisfactory results, a 
stronger solution tending to injure or impair the exquisite delicacy 
and beauty of this organic covering of the shell substance. 
Shells are formed or secreted by the mantle, and are of infinitely 
varied shapes, consistence and ornamentation, and though organi- 
cally connected with the animal, are not vascular structures and have 
therefore no inherent power to repair any injuries they may sustain, 
but if the injuries are situated at or near the margin of the aperture 
they are fully and completely repaired by the 
collar of the mantle, with the pattern and 
ornamentation perfectly reproduced, as in the 
normal shell, so that the reproduced portion 
is often only distinguished with difficulty ; 
if, however, the injuries are remote from 
the margin, the repairs are entirely made by 
the visceral mantle and are thus deficient 
of colour, ornamentation, or epidermis. 
Fig. 17. — An Univalve Shell 
showing injuries remote from 
the aperture, as repaired by the 
visceral mantle. 
Helix nemoralis L., 
Peel, Isle of Man, 
Collected by Mr. E. Collier. 
The class Gastropoda, which includes all our Univalve shells, is 
the most typical and numerous group of the Mollusca, exhibiting 
in the highest degvee the characteristics of the sub-kingdom, and 
showing least affinity in structure with other organisms. 
The shell, with few exceptions, is composed of one single piece or 
valve, hence the term Univalve, though Woodward considers this 
single piece to he homologous with the two shells of a Bivalve united 
above. The shell is usually of a conically tubular form, enrolled 
more or less closely around a central axis, thus diminishing the 
Fig. 18. — A Dextral Univalve. 
Helix neiKoralis L., 
Piperstown, Co. Louth, 
Collected by Miss S. Smith. 
Fig. 19. — A Sinistral Univalve. 
Helix nonoralis m. sinistrorsum F^r. 
Sandhills, I’undoran, Co. Donegal, 
From Miss F. M. Hele. 
space occupied and securing a more portable and compact shell, 
and is, with a few exceptions, coiled dextrally— that is, from left to 
right. It is in most species (piite easy to distingui.sh a dextral 
from a sinistral shell. A simple method is to hold the shell with its 
apex upwards and its mouth directed towards you, when if the 
