INTRODUCTION. 



31 



each other 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, multispiral ; 65, paucispiral ; 66, 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, 



