14 
PECTINIBRANCHIATA. 
the animal, as the operculum 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. PECTINIBRANCHIA TA. 
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. Toxieera. — Head small ; proboscis retractile, 
under the base of the tentacles ; teeth elon- 
