400 SHELLS AND SHELL-FISH. 



indistinct stria upon a shell are called obsolete 'stria, 

 not because they are worn off by accident, but be- 

 cause they are only rudimentary, or very slightly 

 developed. 



Operculum. The hard lid, either corneous or cal- 

 careous, which closes the mouth of many spiral 

 shells, and is carried on the posterior part of the 

 belly of the animal. It generally fits the aperture 

 of the shell, but is sometimes smaller, in which case 

 it is drawn inside until the animal adjusts it to the 

 diminished circumference of the whorl. Nearly all 

 the predaceous tribes have this protection ; but it is 

 never found among such as envelop their shell in the 

 lobes of their mantle. 



Ovate. Not oval, but egg-shaped ; one extremity being 

 thicker and more obtuse than the other. 



Patelliform. Dish shaped, as. the limpets. 



Pectinated. Processes resembling in form and arrange- 

 ment the teeth of a comb, as are the spines of some 

 species of Murex. 



Perforated. As if bored or indented by an awl : the 

 holes or perforations in the ear-shells are of this de- 

 scription. 



Plicated. Applied to any part of a shell which has 

 plaits or folds, either upon the pillar or the external 

 surface: the concentric ridges so common in the 

 genus Venus are also of this description. 



Posterior margin. This term is confined to acephalous 

 bivalves, and is that side of the bosses which contains 

 the ligament. 



Ramose. Spines upon shells which send out others in 

 a lateral direction, are termed ramose or branched. 



Recurved. Turned backwards, in contra-distinction to 

 incurved. The canal of some Strombi and Cerithium 

 are remarkably recurved. The apex of all the patelli- 



