72 



THE GASTROPODA 



is sometimes borne on a differentiated portion of the foot as in 

 Xenophorus (Fig. 134), Pteroceras (Fig. 75), Atlanta (Fig. 141), or on 

 a distinct expansion, which in Natica is reflected over a portion of 

 the shell. An operculum is present in almost all adult Streptoneura, 

 the exceptions being the Docoglossa, the Fissurellidae, the Haliotidae, 



FIG. 47. 



BP 



Natica josephina, fully expanded ; right-side view. I, exhalant orifice ; II, 

 t of propodium reflected on the shell ; IV, tentacles ; V, shell ; VI, poste 

 lected on the shell ; VIII, hind-part of the foot. (After Schiemenz.) 



propodium ; III, 

 rior part of foot 



Gena, Stomatia, the Proserpinidae, Calyptraeidae, Capulidae, Hip- 

 ponycidae, Cypraeidae, Doliidae, Marginellidae, Harpidae, the 

 majority of the Mitridae, many Cancellariidae and Conidae, Oliva, 

 the Janthinidae, etc. But in all these cases, with the exception of 

 the larva of Stylifer, an operculum is present during development, 



n. 



Operculum of Oxygyrus, x 30y external aspect, a.s, attachment surface ; n, nucleus of the 

 opercular spire. 



as may be seen, for example, in the Patellidae, Fissurella, Calyptraea, 

 Janthina, Carinaria, etc. The naked Streptoneura, Entoconcha, 

 Enteroxenos, Pterotrachea, Firoloida also have an operculated shell 

 in the larval stage of development. Among the Euthyneura, on 

 the other hand, only Actaeon and Limacina among the Opistho- 

 branchs, and Amphibola among the Pulmonates, possess an oper- 

 culum in the adult stage, but the great majority, even of the naked 



