394 Proceedings of the Royal Physical Society. 
Wednesday, 25th April 1866. — Professor John Duns in the Chair. 
The following Donations to the Library were laid on the Table, and 
thanks voted to the Donors : — 
1. Geological and Natural History Repertory, Xos. 10, 11, 12, with 
Proceedings of Geologists' Association. 1866. — From the Geologists, 
Association. 2. The Canadian Journal. No. LXI., January 1866 — 
From the Canadian Institute, Toronto, 3. Observations on arrested 
Twin Development. 1866. By P. D. Handyside, M.D.— From the 
Author. 
The following Communications were read : — 
I. The Pearls of the Ythan, Aberdeenshire. By the Rev. James Brodie, 
Monimail, Fife. 
These mussels are, of course, of various sizes, and are found 
sometimes in the running stream, and sometimes in the 
pool. I found them most abundant in those places where 
the bottom consisted of a softish sand, with a gentle current 
flowing over it. The pearls are embedded in a filmy sub- 
stance that occupies the space between the valves and the 
body of the animal. There is not a pearl, even of the 
smallest size, in every mussel ; on the contrary, I have fre- 
quently gathered more than a hundred and have not found 
anything whatever to recompense my trouble. 
I remarked that, generally speaking, the shells collected 
in the streams, where they had been exposed to the tossing 
of the winter floods, most abundantly rewarded the gatherer's 
toil, and that those which were distorted and bore the marks 
of having been broken by violence, were those in which 
pearls were most frequently found. These circumstances 
led me to conclude, that the pearl is produced by some piece 
of broken shell or extraneous matter getting embedded in 
the filmy substance to which I have just referred, and by 
the irritation which its presence produces inducing around 
it a deposit of the same substance as that which lines the 
shell. I was led to regard the formation of the pearl as 
the result of an accident rather than as the effort of a disease, 
as many have supposed The patronage of the 
Queen has made the Scottish pearl to be so much more 
