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THE BLACK OLIVE . 
is one-eighth the natural size. About fourteen species of Tun-shells are known, all inhabiting 
the warmer seas. 
The beautiful Helmet-shells are tolerably thick and solid, and their external surface is 
covered with bold ridges, marking the periodical growth. These ridges are technically called 
“varices.” All the Helmet-shells 
are natives of the tropical seas, 
and appear to prefer the shallow 
waters near the coast. Several of 
these shells are employed by the 
engravers in the manufacture of 
cameos, the differently colored lay- 
ers producing most exquisite ef- 
fects when cut by a judicious oper- 
ator. The colors vary greatly in 
the different species, and some- 
times there is a slight variation 
even in different individuals be- 
longing to the same species. Cam- 
eos, for example, that are cut from 
the Hoened Helmet-shell ( Cassis 
cornuta ) are white, upon a ground of rich orange ; those that are made from the Waety 
Helmet-shell ( Cassis tuberosa) are white, on deep dark red ; the cameos formed from the 
shell of the Reddy Helmet ( Cassis rufa) are saffron-yellow on warm orange. Another 
beautiful species, called the Queen Conch (Cassis madagascariensis), furnishes a white 
cameo on a claret-colored ground. 
HELMET-SHELL. — Cassis glauca . (Small specimen.) 
The next illustration is a dark smooth shell, represented as crawling on the ground, and 
partially enveloped in the spotted textures of the living creature. 
This is the Black Olive, so called on account of the jetty blackness of its exterior, and 
the oval, rounded form, which is not unlike that of the fruit whose name it bears. The genus 
Oliva is a very large one, comprising more 
than one hundred species, and found in all 
the warm and tropical seas. As may be 
seen by the figure, the mantle is furnished 
with two large lobes, that nearly meet over 
the back while the animal is moving, and 
which throw out certain filamentary pro- 
jections, that look very like tenacles in the 
wrong place. The foot is very large — so 
large, indeed, that the shell is partly 
buried in its soft material — and the eyes 
are, as may be seen in the figure, placed 
before the middle of the tenacles. 
Owing, probably, to the great devel- 
opment of the foot and mantle, the Olives are active creatures, gliding about with tolerable 
speed, burying themselves in the sand when the tide leaves the shores on which they are 
creeping ; and if laid upon their backs, they can easily resume their original position by the 
use of the spreading foot. In spite of their elegant and harmless aspect, the Olives are 
predaceous and hungry creatures, and can readily be captured by the simple process of 
tying a piece of meat to a line, lowering it towards the spot where the Olives are creep- 
ing, and hauling it up at intervals, carrying with it the various mollusks that have attached 
themselves to the bait, and do not think of loosening their hold until too late. 
The shell of the Black Olive is beautifully polished and of a deep rich black, through 
BLACK OLIVE .— Oliva mauritana. (Natural size.) 
