38 CEYLON PEARL OYSTER REPORT. 



for the naturalist to have such an animal as this alive under observation. It will be 

 noticed that certain of our figures drawn by Mr. Hornell are taken from the living 

 animal. Some of the photographs are his, others are my own. 



Our pearl oyster and a number of allied " mother-of-pearl " shell-fish belong to the 

 family Aviculid.e, which RiDEWOOD has recently shown to be more nearly allied to 

 the Pectinid.e (scallops) and Spondylid.e, in gill structure, than to the Ostreid^e 

 (the true oysters). They are therefore placed in the order Eleutherorhabda, 

 characterised by the relative freedom of the gill filaments (see description below), 

 while the Ostreid.e and Pinnuxe are placed in the group Eulamellibranchiata. 

 The order Pseudolamellibranchiata, in which the pearl oysters and their allies were 

 formerly placed, has thus been abolished. 



The general characters of the Aviculid^e (pearl oysters and their allies) are as 

 follows : 



The shell is usually inequivalve, the dorsal margin straight, often very long and 

 forming anterior and posterior wings or " auricles," the lateral teeth of the hinge-line 

 are much prolonged and may be inconspicuous, the minute structure of the shell is 

 " cellular"; the mantle lobes are not fused, siphons are absent ; the foot is moderately 

 long, tongue-shaped, with a well-developed byssus gland ; the posterior adductor 

 muscle is very large and nearly central, the anterior adductor is usually absent. The 

 gills may or may not fuse in part with the mantle, gill lamellae plicate and hetero- 

 rhabdid, with both descending and ascending filaments which are held in position to 

 their neighbours by "ciliated discs" placed at intervals along the filaments. 



These and other important points in structure will be more fully described and 

 explained in the pages below and are shown in some of the figures on the plates. 



SHELL. 



The bivalve shell of the Ceylon pearl oyster, Margaritifera vulgaris, Schum. 

 ( = Avicula fucata, Gould), is inequivalve, the left valve being deeper or more 

 convex externally than the right. Each valve is more or less rounded in outline, 

 with a flattened dorsal edge ending in projecting wings or "auricles" in front and 

 behind (Plate I., figs. 2, 3, 4). The dorso-ventral and antero-posterior diameters are 

 much the same, and in a 4-year old specimen measuring 9 "5 centims. in length (dorso- 

 ventrally) the breadth (antero-posterior) is 9 centims. Two shells, both 8 - 5 centims. 

 long, measured respectively 7 centims. and 8 centims. in breadth. Plate I., fig. 5, 

 shows an unusually narrow form. Other variations in the shape of the shell are 

 shown on the plates Plate I., fig. 4 is an unusually straight and fig. 6 an unusually 

 oblique form. In handling some thousands and seeing some millions of these shells, 

 as we have done, one cannot but be struck by the great variation in form and 

 markings. Probably some of the supposed species of Margaritifera are merely 

 varieties of the Ceylon form. 



