INTRODUCTION. 



31 



each otlier all around, so as to form rims, the operculum is 

 described as concentric ; if the nucleus is lateral, or at one 

 side without being spiral it is lammellated ; and when it forms 

 a terminal point, enlarging in the form of a finger-nail or 

 claw, it is unguiculated. In the operculum of a Neritina, 

 there is a lateral process, by means of which it is locked into 

 the columella, the term articulated is then applied. In that 

 of Navicella, there is also a process which appears to radiate 

 from the nucleus, it has therefore been described as a radiated 

 operculum. 



Opercula of Spiral Univalves. 



Fig. 64, muUispiral ; 65, paucispiral ; QQ, concentric ; 67, articu- 

 lated ; 68, radiated ; 69, lammellated ; 70, unguiculated. 



BIVALVE SHELLS. 



Bivalve shells, named Conchacea by Lamarck, are those 

 which consist of two principal portions united to and folded 

 upon each other by means of a hinge. ,The pieces united 

 compose the shell, while each piece separately is called a valve. 

 Considering the bivalve shell as a whole, it will be necessary, 



