14 PECTINIBRANCHIATA. 



the animal, as the operculurn rotates on the foot of 

 the gasteropods. 



2. The operculum of by far the greater number of 

 gasteropods is only formed of animal matter, so that 

 the operculum appears to consist entirely of what 

 constitutes the periostraca or drap marin of the shelly 

 valves ; but the shells of some gasteropods, as that of 

 Aplysia, Bulla, of some land mollusks, and the valves 

 of some bivalves, as Lingula, have only a very thin 

 shelly internal layer, strengthening the thick perios- 

 traca ; while many opercula, like the generality of 

 shells, have a shelly coat deposited on the inner side 

 of the horny periostracal coat, and others have both 

 the inner and outer surface of the animal or perios- 

 tracal part, covered with a shelly coat like the lining. 



ORDER I. PECTINIBRANCHIATA. 



GILLS comb-like, formed of one, or rarely two lon- 

 gitudinal series of lamellae, or very rarely some 

 branched vessels, on the left side of the mantle, 

 over the back of the neck ; the gill cavity is open, 

 the mantle edge being free for the back of the neck. 

 Animal unisexual; shell spiral; operculum usually 

 well developed, rarely absent. 



This order is divided into three sub-orders, 

 thus : 



I. TOXIFERA. Head small ; proboscis retractile, 

 under the base of the tentacles ; teeth elon- 



