74 LIMACID&. 



family ; its members differ from the rest of the Pul- 

 monobranchs in having separate sexes and in their 

 shell being operculate, in which respect they resemble 

 the Pectinibranchs. 



SECTION I. 



Tentacles retractile, usually four ; in some instances the lower 

 pair are rudimentary, in others altogether wanting ; eyes situated 

 on the tips of the upper tentacles or on those of the single pair 

 when the lower ones are absent ; shell in most cases external, 

 complete, spiral ; in others internal and shield-like, or rudi- 

 mentary, consisting only of calcareous granules. 



Family I. LlMAClD^:. 



II. TESTACELLIM:. 

 III. HELICID^E. 



SECTION II. 



Tentacles 2 (with a rudimentary or imperfect lower pair), 

 contractile ; eyes situated slightly behind the inner base of the 

 upper tentacles ; shell spiral, elongated. 



Family IV. CARYCHIID^E. 



SECTION III. 



Tentacles 2, contractile, eyes situated at their outer base ; 

 shell spiral, oval or cylindrical, operculate ; operculum either 

 testaceous or horny. 



Family V. CYCLOSTOMATID/E. 



FAMILY I. 



Body elongated, not spirally coiled ; mantle shield-like, cover- 

 ing the upper part of the back ; tentacles 4, cylindrical, re- 

 tractile, upper tentacles longer than the lower ones ; foot as 

 long as the body and united to it ; jaw arched, smooth or 



