PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
11 
of Natural Science a number of years before I came into 
contact with any of its members. Its journal (The 
Scottish Naturalist) has long been well known, not only 
at home but among working naturalists abroad. Several 
noted Scientific Societies, both at home aud abroad, at 
their own suggestion, exchange their “ Transactions ” for 
the journal referred to. And, I may mention the fact, 
which may not be generally known, that there seem to be 
only three towns in Scotland which appear in the “ Ex¬ 
change Lists” of several British and foreign scientific societies 
—namely, Edinburgh and Glasgow, with their numerous 
scientific bodies, and Perth, which is represented by the 
Perthshire Society of Natural Science. Our Society, 
therefore, occupies an honourable position, and has 
afforded proof of having performed good work up to 
the present time, which is the best earnest it can 
give of its ability and desire to do more in the future. 
It is with every confidence, therefore, in the soundness of 
our Museum scheme, that we have appealed for aid to 
the public, and the response which we have already received 
has been extremely gratifying. To the ladies who have 
so willingly agreed to help us we return our most sincere 
and grateful thanks; and I have not the smallest doubt 
that ere long, with their kindly assistance, we shall be 
able to announce that the much-cherished scheme of our 
late President, Sir Thomas Moncreiffe, has been completely 
realised. 
Mr Andrew Coates said the statement which had just 
been made by Dr Geikie was a most able one, shewing 
clearly the position of the Society; and it was an admirable 
reply to the attack that had been made upon them. He 
thought it was very desirable that the Society should get 
the paper printed and circulated as widely as possible 
among the public, so that they might know the exact posi¬ 
tion of the two Societies, and he moved accordingly. He 
also moved that a hearty vote of thanks be awarded Dr 
Geikie for having made the statement. 
Mr John Thomas seconded both motions of Mr Coates, 
which met with the entire approval of the meeting. 
The following paper was read :— 
“ The Pearl Mussel ( Unio margaritiferus) of the Tay.” 
By Mr Henry Coates. 
During our last session, we were favoured with a 
paper by Dr Buchanan White on “ Cuttlefish and 
their Allies.” These, as doubtless he pointed out, form 
the most important representatives of the first of the five 
great classes into which the sub-kingdom mollusca is 
divided, namely, the Cephalopoda, or “head-footed” 
molluscs. The fresh-water pearl mussel, which forms the 
subject of the present paper, is a well-marked type of 
another of these five classes, namely, the Lamellibranchiata, 
or “ plate-gilled” molluscs, and this is in part my reason 
for bringing it under the consideration of the Society to¬ 
night. Another reason is that it occupies an important 
position in our local fauna, as the Tay has for centuries 
been the chief centre of the pearl fishery in this country. 
Before entering on a description of the mussel, it may be 
well to mention the characteristics which it possesses in 
common with the cuttlefish, and which entitle each to a, 
place iff the same sub-kingdom. Of these the most im¬ 
portant are the following :—Both possess a soft body with 
a complete digestive system isolated from the general body 
cavity, and a nervous sytem with three nerve centres, or 
ganglia. Both, moreover, possess a shell, though with 
this difference, that while that of the mussel is external 
and forms a covering for the body, that of the cuttlefish is 
internal, and merely acts as a support. Leaving these 
points of resemblance—which, it will be observed, are very 
broad, and thus indicate how comprehensive the sub-king¬ 
dom is—we have now to consider the structure of the 
creature before us in greater detail, and shall begin with 
the outer covering or shell. In examining the shell three 
things are particularly to be observed ;—first, that it 
consists of two separate pieces or “valves;’’ second, that 
these two valves are exactly similar to one another in form; 
and, third, that each valve is unsymmetrical—that is, not 
equal-sided. The shell of the mussel is, therefore, said to 
be “bivalve,” “ equivalve,” and “inequilateral,”—three 
peculiarities which are common to the shells of all 
molluscs belonging to this class. The two valves, being 
situated one on either side of the creature, are called 
“ right” and “ left,” and are thus distinguished from those 
of another class of bivalve molluscs, which are more 
properly described as “ upper” and “ lower.” When the 
shell is held with the “hinge” (or line of juncture between 
the two valves) uppermost, and the rounded extremity 
pointing forwards, the valve on the right-hand side of the 
observer is the right valve, and that on the left-hand side 
the left. The rounded end of the shell is the front, or 
“anterior” extremity, and the more pointed the 
“ posterior.” The shell of the mussel is made up of three 
distinct layers, differing from each other in material. 
Lining the interior is a layer of mother-of-pearl or nacre , 
which consists of nearly pure carbonate of lime, arranged 
in extremely delicate plates. These plates, by interrupting 
the rays of light, give to this lining its irridescence which 
we all admire so much. In a fresh specimen, the play 
of colour through every shade of crimson, green, and 
