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 avicidcB, of which our pearl oyster 

 Avicida {Meleagrina) margaritifera is one, the prolonged hinge line, straight 

 at the hinge, is brought in below with a curve that gives it a simiHtude to 

 the wing of a bird, and the sinal ear, though shorter, is also slightly curved 

 in below. In some aviculm this formation is more expressed than in 

 others, so that they are divided into two sections of the long-winged and 

 short-winged aviculce. Avicida macroptera is the type of the former, and 

 Avicida 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 fom*, forms among the challenged spat, sll of which 

 show under the microscope "the prismatic cellular structm'e 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 avicidm, but I 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 ' Natural History of Ceylon ' are therefore 

 wrong. — F.V.T. 



