42 SPOLIA ZEYLANICA. 



What I have called " hinge pearls " always occur near the 

 posterior cardinal ridge or tooth imbedded in the fleshy substance 

 of the central triangular lobe of the mantle which lies between the 

 cardinal teeth. 



Station IV. — On the Nachchikkuda bed round Mutalaippiddi, 

 nearly opposite to a pile of this year's shells on the beach ; depth 

 about 5 feet. From this spot and also from another diving farther 

 up the inlet ofif an outstanding thicket of mangroves near the 

 Sinnakkulam side I recorded twenty measurements. This station 

 evidently corresponds with Mr. Hornell's station 5a (1905), where he 

 found "great abundance of very young Placuna placenta,^^ this being 

 the only part of the Nachchikkuda where he obtained any placunse 

 at all. From this fact we may apparently draw the conclusion 

 that placuna attains the standard size in about 2| years : — 



6 . . 15^ . . 140 . . One pearl. 



7 . . 157 . . 151 . . No pearls. 



8 . . 160 . . 148 . . One pearl. 



9 .. 163 .. 152 .. Nixmerous patent cysts in the 



usual position. 



10 .. 166 .. 151 ... Right valve with Capulids, left 



with adherent sand-tube. 

 Many patent lymphoid cysts 

 in the suspensory membrane 

 of the gill. 



11 .. 166 .. 159 .. Patent cysts present, some 



adjacent to the organ of 



Bojanus. 

 No pearls. 



Do. 

 Capulids and sand-tubes on left 



valve. 



