INTRODUCTION. 



adductor scar to the other, following, at a short 

 distance from the edge, the curve of the lower margin. 



The exact position of the mnbones is more clearly 

 indicated in the second figure than it is in the first, by 

 the small white patch seen on the apex of each valve. 

 The figure also shows the hinge and teeth, which in 

 the Unioiiidce are lateral only ; they are described in 

 the body of the work (see pp. 11-12). 



Cardinal teeth are those placed close to the um- 

 bones (see Sphcerium corneum, p. 3). 



PERIPHERY '• 



UMBILICUS 



OPEROUUJM 



The above three figures show the several parts of a 

 univalve shell. 



