BY E. M. JOHNSTON. F.L.S. 



Umbo. 



The apex, leak or umbo (49 d ) is the 

 point from which the growth 

 commences and is situated near 

 the hinge. The heaks are either 

 straight, as in Pecten, curved as in 

 Venus or Cardium, or spiral, as in 

 Isocardia. 



125 



Example flg. 



No. 



Right and Left Valve. 



When a shell is placed so that the 

 dorsal margin is furthest from the 

 observer, with the exterior side of 

 valve uppermost, the position of 

 the ligament to the right indicates 

 the left valve, if to the left the 

 right valve. 



The lunule is a small cordate or semi- 

 circular impression under and 

 anterior to the beaks. When this 

 appears to the right the valve is 

 right, and vice-versa. 



The sinus or flexure (flg. 49 e ) of the 

 pallial impression (48°) when 

 present is seen under the ligament. 

 When the inside surface is viewed 

 with the dorsal margin furthest 

 from observer, if to the right it 

 indicates the right valve, and vice- 

 versa. Sometimes there are 

 posterior lateral teeth, or laminae 

 of the hinge-margin, and are found 

 on the right or left side accord- 

 ingly as the valve is right or left. 

 Thus the pallial sinus, the external 

 ligament, (49 c ) and the posterior 

 slope (49 a ) are to the right side, 

 (the internal surface being exposed 

 with the beak furthest from 

 observer,) in the right valve (49), 

 and vice-versa. 



Linnasus and the older naturalists 

 were wont to describe the front of 

 the shell as the back and the right 

 valve as the left. 



