UNIVALVE SHELLS. 483 



the basal one, and on the outer lip, distinguishes some 

 shells. 



The interruption may be a mere notch, or a short gutter, 

 or a canal, or rostrum, several inches in length ; and there 

 are intermediate formations to connect these extremes. It 

 may be straight, oblique, or retroverted, open, or covered. 

 It is the sheath of the siphonal process, and is a true index 

 to the character of the latter. 



Spiral univalves are operculated or non-operculated. Adan- 

 son calls the former Sub-Bivalves, a term retained for them 

 by Blainville,* and the propriety of which, in a physiological 

 view, Mr. J. E. Gray has lately attempted to prove. 



The following remarks of Oken on this subject are inter- 

 esting : " The androgynous or bisexual animal is, as 

 a general rule, asymmetrical. The mantle also, or the 

 branchial cavity, obeys this want of symmetry. The 

 branchiae of one side dwindle down ; those of the other turn 

 with the mantle towards the head, and the respiratory aper- 

 ture occurs upon the back. With the one-sided evolution 

 of the mantle, one shell also is only developed, while the 

 other is stunted or placed under arrest. The snail's shell 

 is one of the Bivalve Mollusc's shells, its operculum is the 

 other. This last is stony, horny, and finally is entirely want- 

 ing. It is remarkable that the right shell has been pretty 

 generally perfected, while the left dwindles down into the 

 operculum ; all the snail's openings are therefore on the 

 right side, such as the anus, with the orifices for the escape 

 of the ova and semen. "f I may just observe, that opercu- 

 lated univalves are not the nearest relations to the bivalves 

 in the structure of their living tenants. 



The operculum is a horny or calcareous plug designed to 

 close the aperture of the shell. It belongs, with very few 

 exceptions, to the shells of pectinibranchial Gasteropods ; 

 but indifferently to the zoophagous and phytivorous fami- 

 lies amongst them. " It has been observed that shells with 

 toothed columellae never have opercula ; but many instances 

 may be produced to the contrary." J. E. Gray. 



The operculum is affixed above the foot of the animal, on 

 a circumscribed portion of the mantle, distinguished by its 

 denser texture. As its position varies somewhat, we find 

 that when the animal is in motion, the operculum is some- 

 times placed near the tail, as in the Cones ; sometimes near 



* Man. de Malacologie, 229. But compare his opinion in this place with 

 his objection to its justness at page 103 of the same work, 

 t Physiophilosophy, p. 524. 



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