178 First Report on Economic Zoology. 
in being differently coloured, the dark lines of colour radiating as in the 
drawing from the convexity to the contour. Again, it is only the concave 
part of the shell that is coated with glistening nacre, the broad deflected 
margin being dull. It is not so in the pearl oyster, in which the nacre 
comes close to the margin. In the aviculce, of which our pearl oyster 
Avicula (. Meleagrina ) margaritifera is one, the prolonged hinge line, straight 
at the hinge, is brought in below with a curve that gives it a similitude to 
the wing of a bird, and the sinal ear, though shorter, is also slightly curved 
in below. In some aviculcz this formation is more expressed than in 
others, so that they are divided into two sections of the long-winged and 
short-winged aviculce. Avicula macroptera is the type of the former, and 
Avicula heteroptera and A. crocea are illustrations of it ; of the latter 
A. margaritifera is the type, but still has the peculiarity distinctly present. 
In the challenged spat it is wholly absent. At the same time, however, 
that it is said in Reeve’s “ Conchologia Iconica ” that this feature is 
always present in the aviculce , it is not shown in the small shell of 
A. vexillum, figured magnified in this work, which, as far as the drawing 
goes, has a general similitude to the challenged spat and has against it the 
remark “ Habitat, Ceylon (in deep water), Gardner,” but beyond this the 
text description, though very brief, hardly tallies, and there are to my 
thinking three, if not four, forms among the challenged spat, all of which 
show under the microscope “the prismatic cellular structure of shell 
found in most of the aviculce ” (Carpenter). My belief is that they have 
been so long sailing under the false colours of being the pearl oyster spat, 
that they are unnamed and seemingly mature aviculce , but 1 am not 
concerned to name them ; all my contention for the purposes of this report 
is that they are not pearl oysters.* 
This is pointed out as having an important bearing on the supposed 
disappearance of young pearl oysters from certain beds. 
* The figures given in Tennent’s 1 Natural History of Ceylon ’ are therefore 
wrong. — F.V.T. 
