OF THE GULF OF MANAAR. 
107 
different flanges, and no algie adhering to it. And it has 
the umbones more anterior or advanced beyond the hinge 
line. It is colored differently, dark lines radiating from the 
convexity to the contour ; and its habit is different, in that 
it adheres to weed, while A. fucatio affects rock. 
Further to facilitate the observation of the difference in 
the form of the shells the minute aviciila vexiUum which 
attains only a quarter of an inch in breadth has been photo- 
graphed and magnified to the size of A. fucata, when the 
differences are emphasised and observable beyond dispute. 
Furthermore, any lingering contention that such changes 
of form are referible to development is effectually disposed of 
by the discovery of the true spat of Acicala fucata at an age 
when it is much smaller than a quarter of an inch across, and 
it has markedly all the characteristics of a half-grown pearl 
oyster. If it differs at all from its half-grown form it is in 
having the spatulated spines comparatively more expressed, 
and the slight convexity still slighter in comparison, so that 
in both respects it opposes itself to similitude with Avicula 
vexillum. I have seen them alive and have them in spirits 
of progressive sizes from one-twelfth of an inch from hinge 
to contour rectangularly to half an inch, and after that the 
connecting progress is easy to trace, and the very smallest 
had all the characteristics of a pearl oyster of a year old from 
the prismatic cellular structure of the shell, down to the 
spatulce foliations and algae thereon. 
The Age. 
The age of the oyster is considered a matter of some 
practical importance, because it is held that our pearl 
oyster lives for 7 years and then dies, consequently that the 
fishery must be conducted before they die, or the pearls 
will have dropped out with the dead decaying animal. It is 
also held that the pearls are too small to be worth taking till 
the fish is more than 4 years' old, so that when a Superin- 
