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, 
Vv \v 
Fic. 47. 
Natica josephina, fully expanded ; right-side view. I, exhalant orifice ; Ii, propodium ; ITT, 
part of propodium reflected on the shell; IV, tentacles; V, shell; VI, posterior part of foot 
reflected on the shell; VIII, hind-part of the foot. (After Schiemenz.) 
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, 
as 
Fic. 48. 
Operculum of Oxygyrus, x 30, external aspect. a.s, attachment surface ; ”, nucleus of the 
opercular spire. 
‘as may be seen, for example, in the Patellidae, Fisswrella, Calyptraea, 
Janthina, Carinaria, ete. 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 Actacon 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 
