# What Are Spiral Shells Called?

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The nautilus shell is the image we generally picture when talking about a spiral shell. This shell is not just an example of a spiral; it is a mathematically perfect spiral, with proportions known throughout history as the Golden Ratio.

## Nautilus

• The nautilus is part of the cephalopod family, which includes squid and octopus, but the nautilus is exceptional in that it grows a hard shell which it creates sequentially as the animal grows larger, moving into new chambers and sealing the others off with a wall of mother-of-pearl. The sealed chambers then serve to keep the larger shell buoyant.

## The Golden Ratio

• What is so special about the nautilus shell is its mathematical perfection. The Golden Ratio is defined as the ratio resulting from a line divided so that the length of the longer segment is 1.618054 times the length of the shorter segment. A spiral forms a logarithmic spiral as it builds on itself getting proportionally larger in this exact ratio of 1.618054 times the smaller, previous shell chamber. The Greeks were the first to call this mathematical equation found throughout nature the Golden Ratio.

## Other Spiral Shells

• The nautilus is the perfect, spiral shell, but other shells follow a spiral shape as well, including auger shells and turret shells. Rather than being round, flat spirals, both of these shells spiral upwards to a point.

## References

• Photo Credit nautilus shell image by Photosani from Fotolia.com
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