GASTROPODA. 13 
and the external shelly coat is generally brightly coloured and 
highly ornamented. Several very pretty species are found on our 
own shores. The opercula of Turbo petholatus (Fig. 6), from the 
Indian and Pacific Oceans, are frequently mounted in gold and 
silver as scarf-pins, ear-rings, &c. 
The Nerites are mostly found in tropical countries, and, like the Cases 
Winkles, are very strongly made, to resist the force of the breaking 18-20. 
waves. The Neritinas are partly found in the sea, and partly in 
fresh water, and are less solid shells. The third section of Neritidm, 
the Septarice, are shaped very much like Limpets, except that the 
apex is at one end instead of central. They are, however, very 
different animals, and furnished with a shelly operculum imbedded 
in the foot. 
Order 2. — Pectinibranchia. 
In most cases the molluscs of this order have an attached Cases 
monopectinate gill and a single osphradium. A few are fresh or 22-94 - 
brackish water forms, but the majority are marine. 
The "River-Snails" (Viviparidce) might be termed freshwater Case 22. 
Periwinkles, as the animals of both are very similar. The true 
Fig. 7. 
V 
The Common British River-Snail (Vivipara vivipara). 
a, head ; b, tentacles ; c, eyes ; d, foot ; e, operculum. 
Viviparm are viviparous. They are rather sluggish, and found at 
the bottom of ponds and rivers feeding on decaying animal and 
vegetable matter. 
The Cyclophoridw are land-shells, which, however, cannot properly Cases 
be considered true lung-breathers like ordinary snails. They have 23 ~ 25 - 
not the closed lung-chamber of the Pulmonates, their eyes are 
