104 Marine Shells of the Western Coast of Florida 



cuticular development upon the foot and is placed precisely over the 

 end of the strong muscle which attaches the mollusk to the columella 

 of its shell. Ihe operculum more or less completely closes the shell's 

 aperture when the mollusk is retracted within, and it ma}^ be held 

 in this position for a long time if the animal is alarmed or exposed to 

 drying. Generally the operculum is adapted to the shape of the aper- 

 ture. In structure, it may be corneous and of a brown or yellowish 

 color, or calcareous, when it is usually white or tinted with pinkish, 

 brownish, or green coloration. Opercula vary much in shape and 

 thickness, they are rudimentary in some genera and in others entirely 

 lacking. There is a nucleus from which growth proceeds concentri- 

 cally or excentrically; the position of the nucleus differs in different 

 genera and is of importance m classification. 



The usual turn of the gasteropod shell is to the right — dextral 

 — showing the aperture on the right side when the shell is held with 

 the apex upward. Some species are normally sinistral — the spiral 

 turns leftward from the apex, placing the aperture on the left side 

 when the shell is held apex upward. Species which are normally 

 dextral may develop sinistral abnormalities, and species which are 

 normally sinistral sometimes present a right spiral. 



Anomalies of the gasteropod shell are not uncommon. They 

 appear more frequently than among the shells of the Pelecypoda or 

 the Amphineura. Busycon "contrarium" often shows deformities 

 and duplications of the canal for its entire length, but rarely any 

 deformity of the body of the shell. Embryonic shells, still in the 

 egg capsule, have been observed by Perry which show the same de- 

 formities as those seen in semiadult and adult specimens; this does 

 not seem to have been previously noted. Many individuals show de- 

 formities of the whorls which are due to the inclusion of barnacles 

 or boring mollusks. 



Albinism is observed in Busycon, Murex, Strornbus, Oliva, 

 Urosalpinx, and Muricopsis, and the operculum of such a shell is 

 of a much paler color than the normal form. 



Many gasteropods leave trails in sandy sea bottoms which are 

 as distinctive to the experienced collector as trails m the snow are 

 to the hunter. Natica, Polinices, Melongena, Busycon, Fasciolaria, 



