456 TRANSACTIONS LIVERPOOL BIOLOGICAL, SOCIETY. 
occurrence is the development of a furrow proceeding 
from a small depression on the surface of the embryo, 
which is the early shell gland. 
A velum is developed as a circular ridge covered with 
long cilia, and encircling the embryo between the 
blastopore and the shell gland. The stages following 
have not been observed in P. opercularis, but probably 
there are one or two long apical cilia developed in the 
centre of the velum, and the embryo elongates in the 
direction of the axis of these cilia, though in the stages 
which Fullarton observed with the shell valves developed, 
there were neither apical cilia nor flagellum present. A 
trochospere larva is thus formed, with a velum but no 
shell. Later the mantle folds are developed and two 
shell valves, and the veliger stage is entered upon, 
locomotion still being carried on by the velum, which 1s 
extended beyond the margins of the valves and is retracted 
by two velar muscles. A single adductor, the anterior, 1s 
developed, and the alimentary canal formed; the mouth- 
opening being just posterior to the velum, and the anus 
close to the mouth. 
ECONOMIC IMPORTANCKE. 
Pecten opercularis occurs in large beds which are 
found in many localities round the British Isles. In 
most of these places little use seems to be made of the 
scallops, though they might be of considerable value both 
as food and bait. ‘The species occurs in such abundance 
in the Firth of Forth that an important industry has 
arisen, which once employed a considerable number of 
hands, though in recent years the number has diminished. 
In Scotland, P. opercularis is known as the “clam,” 
though it must be remembered that this name has been 
