UNIVALVE MOLLUSCANS. 207 



cuttle-fish and nautilus tribe constitute Lamarck's 

 Class of Molluscans. The latter author divides 

 his Class into five orders, four of which belong 

 to the tribe I am considering. 



1 . Pteropods (wing-footed) ; furnished with 

 organs only for swimming and sailing. 1 



2. Gastropods (belly-footed); body straight, 

 never spirally convolved ; a muscular foot for 

 creeping under the belly. 



3. Trachelipods (neck-footed) ; greatest part of 

 the body spirally convolved, always inhabiting 

 a spirivalve shell ; foot free, attached to the 

 neck, formed for creeping. 



4. Heteropods (diverse-footed) ; no coronet of 

 arms; no subventral, or subjugular foot; fins, 

 one or more, not disposed in pairs. 2 



As the Cephalopods, forming Lamarck's fourth 

 Order, may be regarded rather as constituting a 

 larger division or Sub-class of the Molluscans, 

 than an Order, I shall consider them in a sepa- 

 rate chapter. 



1 . Proceeding from one of the above Orders to 

 another, I shall select such individuals, belong- 

 ing to it, as appear to exemplify the great attri- 

 butes of their Creator, either in their structure, 

 forms, habits, or instincts. The animals of the 

 first Order, like the long celebrated Argonaut 

 and Nautilus, enliven the surface of the ocean 



i PLATE V. FIG. 6, 7. 2 FIG. 8. 



