GASTROPODS. 
373 
one central or rhachidian, and one lateral on each side; the latter, however, are 
wanting in certain groups. The respiratory siphon is rather long, fitting into the 
anterior canal or notch of the aperture. The eggs of these molluscs are deposited in 
strong leathery capsules, each capsule often containing a large number of ova. The 
genus Yetus, however, which is viviparous, produces a comparatively small number 
of young. All have shells, which in nearly all cases are capable of sheltering the 
entire animal. The principal families in this group are the Olividce, Harpidce, 
Marginellidce, Volutidce, Mitridce, Fasciolariidce, Turbinellidce, Buccinidce, 
Nassidce, Columbellidce, Muricidce, and Corolliophilidce. 
The Olives ( Olividce ) are mostly cylindrical shells, often beautifully orna¬ 
mented with markings of various patterns, and always having a highly glossy or 
enamelled appearance. The aperture is narrow or notched in front, and the 
columella is not strongly plaited as in some of the families which follow 
(Marginellidce ), but there 
are numerous, slight, 
oblique folds, some of 
which pass round the base 
or anterior part of the 
shell, which is thus marked 
off from the rest of the 
surface into a distinct area. 
The animal has a very 
large foot, capable of 
covering the shell to 
a 
black olive, Oliva maura (nat. size). 
great extent, pointed 
behind, and with its anterior portion (propodium) divided into two lobes. The 
head is small, with pointed, slender tentacles, and the eyes about midway along 
their outer sides. The mantle is produced in front into a slender appendage, 
which protrudes with the breathing-siphon through the notch of the aperture; 
posteriorly it terminates in a thread-like process, which passes up the channelled 
suture of the spire. There is no operculum. The olives are very active, and burrow 
in the sand in search of bivalves; and are sometimes seen in countless numbers, 
the sands at low tide for miles being covered with them. Although about one 
hundred and fifty to two hundred species, mostly tropical, have been described, 
a few extend to more temperate seas as far north as Japan, and southward to 
Patagonia. The olives are frequently used in the manufacture of shell-ornaments. 
The genus Olivella differs from the typical Oliva in having neither tentacles nor 
eyes, and the spire of the shell is longer and more pointed. Ancilla, another 
important genus, comprises a number of polished species, which are nearly always 
of a uniform white, yellow, fawn, brown, or reddish colour, and without the 
markings of the olives. 
The family Harpidce contains but a single genus, the well-known harp-shells 
(j Harpa). These are strong, broad, ventricose structures, highly coloured, and 
adorned with numerous curved ribs, running parallel with the outer lip of the 
aperture. The columellar margin is smooth, and covered with a thin brilliant 
callus. The animal has an enormous foot, which is not wholly retractile within 
