KEY TO PLATE IV 



First line- Full-grown larva of -34 mm., from the right side, from 

 behind, and from the left. Magnified 100 diameters. 



Second line: Spats of 1, 3, 6 mm., from the right, showing the larval 

 shell at the umbo, magnified 10 diameters. 



Third line: Oyster of 60 mm., round variety, natural size, from the 

 left side, and from the edge, attached to a periwinkle which it has out- 

 gi'own. 



(Drawn by Mr. F. W. Parker) 



