8 
they emerged from the egg they were known as the larve, and if 
they had not a shell at the time of birth, they soon began to form 
one. In this condition they could swim about in water, and on 
their back they formed the shell, while the ventral surface became 
the foot or creeping disc. But changes soon and rapidly took 
place, and the visceral sac and the shell became coiled in a 
nautiloid form, and at this stage it depended in the case of 
univalve shells as to which form the shell would assume, whether 
a dextral or right hand, or sinistral or left hand. 
The visceral sac was covered with an integument known as the 
mantle, the epidermis of which was filled with secretive glands, 
and these produced the materials for the formation of the shell. 
The formation and deposition of the shelly material was then 
shown on the screen, as also were microscopic fragments of the 
materials by some five or six slides. 
The shell consisted of three distinct layers ; the inner layer 
having a nacreous or mother of pearl texture. Next to that was the 
portion known as the porcellanous or prismatic layer. It was 
composed of large pallisade-like prisms which were placed side by 
side. The texture was something like the enamel of teeth. It 
contained the colouring pigments which gave to most shells the 
charm they possessed to almost everybody. The outer surface of 
the shell was covered with a horny cuticle, known as the epidermis 
or periostracum. Both it and the prismatic layer were secreted 
by the free edge of the mantle. It was composed mainly of 
chitinous substance and served as a protection to the external 
surface of the shell, and in many instances acted as a varnish by 
throwing into relief the colouring beneath. The mantle, in the 
case of bivalve shells, was developed on two sides of the animal 
into aright and a left lobe. In the univalve shells it was continuous. 
The skin of the mantle consisted of three parts: a one layered 
epidermis of columnar cells, a highly vascular connective tissue 
containing abundant muscular fibres, and the ciliated layer of cells. 
The shell was composed of carbonate of lime and conchyolin. 
As a rule it was hard and calcareous, but some were delicate in 
structure, horny, and flexible. The periostracum varied greatly in 
its different qualities. (A slide was shown in which it was seen 
on shells, as long course fibres growing, as it were, out of the 
shells, while other specimens showed it produced to a great length 
beyond the end of the valves and forming a strong leathery casing 
for the siphonal tubes of the mollusc). 
In some shells it was closely adherent and even connected to 
the valve, while in its coarser forms it would peel off after the 
death of the mollusc. 
The lecturer pointed out that it must not be confused with 
the epiphragm which was a kind of lid or covering made by many 
kinds of snails to the opening of the shell; nor yet with the 
operculum, which was a horny formation in some, and of a shelly 
EO 
aS 
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