The gui-xiang, also known as the kuei shen (variously translated as 'somber warrior', 'spiritual being', and 'black tortoise'), is the term for the Chinese spiritual tortoise. Gui-xian is a tortoise, not a turtle, the difference being that a turtle lives mainly in the water, whereas a tortoise spends most of its life on land. As in many cultures, the tortoise is a symbol of wisdom, longevity, life, love, the world, and divination; however, the gui-xian is most strongly associated with wisdom, long life, and divination.
According to Chinese myth, the gui-xian lived mainly in the sea, and therefore was the king of all mollusks and sea creatures.
Because of the tortoise's segmented back, it has a square-ish section in the middle of its shell. This is often called the 'magic square'. The Chinese often used a dead tortoise's shell and innards to tell the future, and the magic square was the most important part in successfully divining the information. In addition, gui-xian presided over the Chinese emperor's destiny, and was consulted when divining his future.
The tortoise was a symbol of wisdom because of it's long life span. The tortoise could live thousands upon thousands of years; it was considered most wise after it reached the age of 1,000 years, at which point it could speak human languages. The tortoise is also a symbol of happiness to the Chinese people.
As part of the si ling, the four Chinese spiritual animals, the tortoise is a highly respected creature. The other creatures are the qilin, or unicorn; the long, or dragon; and the feng-huang, or phoenix. When the Earth was created, the heavens were split into four quadrants--East, West, South, and North--and each was protected by one of the si ling. That particular si ling also represented the corresponding compass point. The gui-xian protected the North; it was also associated with the element earth, the color black, and the season Winter.
While the tortoise is considered to be a spiritual animal, is is separate from the other si ling in that it is not a mythical creature; meaning, that while the characteristics attributed to it are mythical, the actual creature exists, whereas qilins, longs, and feng huangs do not exist at all except in people's imaginations (though Glowing Fish would probably not back me up on this).