ANATOMY OF THE PEABL 0Y8TEB. 39 



The shell is very thin, about L'5 millimB. over the greater part of its extent, and is 

 lined by an exceedingly brilliant layer of nacre or mother-of-pearl. The outside ol 

 the slid] is usually marked by o' or 8 radial bands of dark red or reddish-brown on a 

 pale yellowish ground. These colours are brightest and best seen in young specimens, 

 .is iii the older shell they become dulled and obscured on the outside by incrustations 

 and growths (Plate II., fig. 2). They are, however, usually visible even in lai _ 

 specimens on the inside of the valve on the ventral margin (see fig. 1). The outer- 

 surface in the young shell Lb also marked by concentric ridges or projecting 

 imbricating lamellae, which grow out at intervals to form spatulate or fingerdike 

 pro Plate V II., fig. 14), which maybe over 12 milliins. in length. These 



sometimes seen in the old shell (Plate I., fig. 2). The layer of nacre ends from 

 10 milliins. to 15 milliins. back from the tree edge of the shell (Plate I., fig. 4), and 

 at that point the shell in a well-grown specimen is about 1 millirn. in thickness. 

 From this the shell thickens gradually towards the dorsal edge, reaching an average 

 of 2 millims. at about the point of greatest convexity of the vab 



In thick shells it may be as much as 3 millims. at this point, and an examination 

 of the section shows that this increased thickness is due entirely to the nacre, which 

 mav be 2 milliins., while the prismatic laver is only 1 millirn. Over the greater part 

 of the shell these two layers are of equal thickness, say from 0'5 millim. to 1 millirn. 



i. < in the anterior ear of the shell, forming the side of the byssal notch, ther< 

 a thic-kening of the shell up to about 5 millims. ; and at the hinge, in the mid-dorsal 

 line, another local increase may reach to 8 millims. 



At the ventral edge the shell beyond the lining of nacre gradually decreases m 

 thickness, and in a rapidly growing shell the free edge is flexible and horny in 

 c insistence, being composed of periostracum and a thin layer of prismatic shell still 

 imperfectly calcified. Two shells from Kondatchi Paar (17th November, 1902), 

 measuring 7 centims. in length, have this delicate margin, free from nacre, extending 

 up to 2 centims. in width, not including the processes at the edge (Plate I., fig. 



< )n Plate I., figs. 1 and 2 show the contrast between an unusually smooth and an 

 unusually imbricated shell; figs. 3 and 4 show variations in the development of the 

 nacre and the margin, and figs. 5 and G marked differences in the shape of the shell. 

 The " auricles'' and the byssal notch also vary much in their development, as may be 

 seen from the figures. 



The hinge line is a narrow ridge running along the greater part of the straight 

 dorsal edge and in contact with its fellow of the other valve, but not conspicuously 

 cut up into projecting teeth. Much elongated, narrow, ridge-like lateral teeth are 

 present. Outside the middle third of its length is the large black elastic ligament 

 (see Plate I., fig. 4). It may measure in an adult shell about 20 millims. in length 

 and 5 millims. in breadth. The ligament serves to open the shell, and so is anty_ 

 nistic to the adductor muscle. Alongside the ligament and extending from the hinge 

 line upwards to the umbo (the most prominent point of the valve, placed in front i <\ 



