THE OPERCULUM. 47 



when taking the direction of the axis, and revolving (or less 

 properly transverse) when they follow the spiral. In the latter 

 case the}^ are also sometimes spoken of as longitudinal^ with 

 reference to the whorls only and not to the entire shell ; but to 

 avoid confusion it is becoming usual to consider their direction 

 with reference to the shell. 



The following are the principal varieties of sculpture : — 

 Canaliculate, suture profoundly channeled. 

 Cingulate, encircled by revolving ribs. 

 y^ Carinate, revolving sculpture prominent, sharp. 



Sulcate, encircled by channels. 

 / Plicate, costate, ribbed, sculpture longitudinal. 

 ^^^^^^JS'odosely,tuberculately,granosely plicate, when the ribs are 

 broken up into tubercles or granules ; mostly caused by 

 the intersection of revolving sculpture. 

 y Striate, covered by fine close lines, either longitudinal or 

 revolving. 

 Punctate, pitted. 



Frequently punctate-striate are combined, as in Mitra. 

 Granulate, nodose, tuberculate, covered with nodules of 



small or large size, but not ribbed. 

 Muricate, spinous, echinate, when the nodules are sharp- 

 pointed. 

 Decussate, cancellate, longitudinal and revolving sculpture 



crossing at right-angles. 

 Reticulate, sculpture not crossing at right-angles ; irregu- 

 larly decussated. 

 Clathrate, longitudinal and revolving lines both distant, 

 forming a pattern somewhat like the iron bars of a prison 

 window. 



The last two terms are often used improperly as synony- 

 mous with decussate or cancellate. 

 Coronate, the upper part of the whorls having a series of 



revolving tubercles or spines. 

 Yaricose, when the external thickening of the margin of the 

 aperture occurring in some shells during rest-periods, is 

 not absorbed away when growth is resumed, but remains, 

 crossing the whorls at regular intervals. 



THE OPERCULUM. 



This is present in most gastropod mollusca, particularly in 

 those provided with spiral shells ; it also occurs in the larval 

 stage of many nudibranchiate and opisthobranchiate mollusks, 

 which are then provided with a temporary shell. The pulmon- 

 iferous land and fresh-water gastropods are inoperculate. 



At the ending of the columellar muscle in the dorsum of the 



