GASTROPODS. 
39 1 
like most other gastropods. In the family of the naticas ( Naticidce ), which is 
numerous in both recent and fossil species, the shells are more or less globular 
or ear-shaped, and the 
animals remarkable for 
the enormous develop¬ 
ment of the foot, which 
is used as a plough to 
drive through the sand 
in search of other mol¬ 
luscs—mostly bivalves 
—upon which they 
feed. The foot is so 
large as almost to 
conceal the shell, and 
the front part of it 
(the propodium) hides 
the head, which has no 
visible eyes. The 
shells are quite closed starfish with Thyca; a, Shell of latter enlarged, 
in some of the groups 
by a paucispiral operculum, which is cither horny or shelly. Naticas are found in 
all parts of the globe. The largest living species (Lunatia keros) occurs on the 
eastern coast of North America, but some 
of the more highly-coloured forms are 
found in tropical seas. About half a 
dozen species occur in Britain. The 
spawn-cases are curious objects, resem¬ 
bling a broad sandy strap coiled round 
like a horseshoe. 
b, A bubble, drawn, somewhat too large, about to be 
joined to the anterior end of the float; c, Shell; 
l. Float; p, Foot; t, Head. 
SECTION PTENOGLOSSA. 
In this unimportant group of Pectini- 
branchs—which includes only the families 
Ianthinidce and Scalariidce —the radula 
consists of numerous rows of pointed 
teeth, arranged in cross series forming an 
angle in the middle. There is no central 
or rhachidian tooth. The violet sea-snails 
(Ianthinidce) have thin trochiform shells, 
adapted for a pelagic life. These are 
mostly of a violet colour, the base or 
underside, which is turned upwards and 
exposed to the light when the animal is 
swimming at the surface of the sea, being 
more deeply tinted than the rest of the 
