268 



THE OPERCULUM 



and Pcdelhi liave reached the stage at which the opercuhim lias 

 been lost entirely. In Navicella, besides l)econiing degraded, the 

 operculum has actually become |)ai'tly internal, and ajijiarcully 

 serves the purpose of separating the viscera from the upper 

 part of the foot, something like the shelly plate in Crepuhiht. 

 This explains why tlie operculum in this genus is polished on 

 ))oth sides.' 



Some authors have imagined that the opercuhuu is homo- 

 logous (a) to the second valve in Pelecy})oda, {h) to the hyssus. 

 It dilfcrs, however, morphologically from the former in tiie 

 essential ])oint of not being produced l)y the mantle, and from 

 the latter in not lieing produced l)y a special gland. 



As regards shape and formation, the operculum has usually 

 a more or less well-marked nucleus which may be central (e.g. 

 Livuna), subcentral (Anqmllaria), lateral (^J'Ki'jrura), or terminal 



Turbo 

 (Sanuaticus) 



Livoiia 



Tiu'lio 

 (Callopoiiia) 



Fig. 182. — Various forms of open-ula 



Anipnllaria 



Xatiea 



(Fijo'ida). As a rule, l)oth the inner and outer surfaces are fairly 

 flat, but in Torinia, Cyatho2-)oma, and Ftcrocyclus the outer sur- 

 face is elevated and conically spiral, in some Turho (e.g. Sarmati- 

 ciis) it is covered with raised tubercles reseml)ling coral, while in 

 others (e.g. Callopoma) it is scored with a deep trench. Aalo- 

 2wma, a land genus peculiar to Ceylon, has a paucispiral oper- 

 culum with hollow whorls, deceptively like a PlanorUs ; it fits 

 over the aperture instead of into it. In Livona and most 

 Trochidae the operculum is cartilaginous and multispiral. In 

 Stromhus it is narrow, curved, and often serrated like a leaf 

 on one of the edges; in Conus it is narrowly oblong and rather 

 featureless; in Littorina, paucispiral and always cartilaginous. 



^ .T. E. Gray, ridl. Trans. 1833, p. 812. 



