HORNELL ANATOMY Of PLACUNA 53 



may appear at the anterior end of the dorsal margin ; at the posterior end a single and 

 larger lobe is usually developed. In rare and extreme cases one of these " ears " or 

 lobes at each end of the hinge line, growing out at right angles to this line, becomes so 

 pronounced that they include the hinge region in a deep bay. In adult shells these 

 ear-like lobes usually become broken off, but remnants of them may generally be 

 traced. 



The anterior, ventral, and posterior margins of the valves in undamaged adults are 

 typically entire ; such a margin is frequently seen in immature individuals, but a large 

 proportion of the larger shells show irregularities due in the first instance to injuries 

 inflicted by fishes. Plate I., fig. 2 (A and B), shows two extreme instances. In the 

 former (A), several injuries of old standing are recorded in the irregular or " bayed " 

 appearance of some of the old concentric growth-lines, while the ventral edge shows a 

 large one of recent infliction, as indicated by the clearness of the edges of the semi- 

 circular gap resulting from a fish's bite. As in the true pearl oyster (Margaritifera 

 vulgaris), reparation of such injuries is extremely rapid, and so long as the bite affects 

 no organs or tissues except the mantle edge, Placuna placenta possesses great 

 recuperative powers. Such injuries have economic interest, as they constitute an 

 effective and necessary factor in the life-cycle of the cestode and trematode parasites 

 whose larvae form the nuclei of many Placuna seed pearls. 



The outer surface of the shell is marked by distinct concentric lines of growth, 

 consisting of slightly projecting imbricate lamellae, the margins of which are rendered 

 minutely uneven or roughly dentate by the presence of numbers of short, closely set 

 and very delicate processes, spatulate or finger-like in form, and rather ragged in their 

 irregularity and wear (Plate I., fig. 2). 



These concentric rows of minute processes impart to the shell a certain roughness 

 to the touch ; they occlude many particles of mud, and this is apt to give the shell a 

 dirty brownish colouring, and the false appearance of having a rough superficial 

 periostracal investment. 



The two valves of the shell are united dorsally by a short hinge ligament 

 coincident with the greater part of the straight dorsal edge. On the dorsal edge the 

 ligament is straight, while on the ventral two very long linear projections, and a 

 number of short, narrow dentate ones are seen, corresponding in position and size 

 with a similar number of projecting teeth arming the hinge line of the opposite right 

 valve (Plate II., figs. 2 and 3). So relatively strong is the ligament that a small 

 portion of the dorsal margin of the right valve is usually broken off, and remains 

 attached to the ligament when the valves are forcibly separated. 



In very young specimens the hinge ligament is dark golden brown ; with age the 

 colour deepens, till in the fully mature it becomes a brownish black with a bronze green 

 lustre. 



On the right valve two very prominent cardinal teeth arise close together at the 



F 2 



