MARINE MOLLUSCS 221 



Pinna, so called on account of the fins or wings on the dorsal side 

 of the shell. In this group the shell is more or less wedge-shaped, 

 with equal valves, and the umbones are quite at the anterior end, 

 while it is blunted and gaping at the other end. The hinge has no 

 teeth. The margins of the mantle are doubly fringed, and the 

 byssus is extremely powerful. 



The Common Pinna (P. pectinatd) is a very large mollusc, 

 sometimes measuring a foot in length, and is very abundant off the 

 south-west coast, where it moors itself vertically at the bottom of 

 the water with the pointed end buried, and the broad end gaping 

 widely so as to expose its body. It has been stated that fishes are 

 frequently tempted to intrude into the open shell for the purpose of 

 devouring the animal within, and that they are immediately crushed 

 by the sudden closing of the valves, which are pulled together by 

 two large and powerful adductors. 



We have already referred to the little Pea Crab that inherits the 

 shell of the Pinna, living permanently in the mantle cavity of the 

 animal. 



The last family of the Lamellibranchs is the Ostreidce or Oysters, 

 of which the edible oyster may be taken as a type. In this 

 group the shells are frequently unequal, and they lie on one side 

 either free or adherent to the surface below them; the hinge is 

 usually without teeth. The mantle is quite open, the gills number 

 two on each side, and the foot is either small or absent. 



The Edible Oyster is a type of the typical genus Ostrea, its 

 scientific name being Ostrea edulis ; and as this mollusc may be 

 readily obtained at any time, it is a convenient species for the study 

 of the general characteristics of its family. Its shell is irregular in 

 form, and the animal always rests on its left valve, which is convex, 

 while the upper or right valve is either flat or concave. The lower 

 valve is also thicker and laminated in structure, and is attached to 

 the surface on which it rests. On examining the interior we find 

 that the shell is somewhat pearly in appearance, and that the edges 

 of the mantle lobes are finely fringed. The gills, too, are united 

 with each other and with the mantle on the posterior side, thus 

 forming a distinct branchial chamber. 



Oysters are found on banks at the depth of several fathoms, 

 where they spawn in early summer, and the fry or spats are 

 collected in large numbers and transferred to artificial beds or 

 tanks, where they are kept in very shallow water so as to be easily 

 obtainable when required for food. It is interesting to note, how- 



