An update on the saga of the endangered sea turtle;

WHAT do a fashion-conscious Italian and a starving Bengali have in common? According to author Ar him Carr, they share a lust for sea turTles - the one for fancy shoe leather and the other for food.

The net result is a continuing threat of extinction for Lepidochelys olivacea , known commonly as the olive ridley.

Since ''So Excellent A Fishe'' first posed the riddle of the ridleys 17 years ago - the mystery of where these animals go when they aren't nesting - much has been learned. But this and other riddles remain.

As long ago as the 17th century the need to protect ''s was recognized by the Bermuda Assembly, but still the animal is exploited to the danger point. Each time a new nesting ground is discovered there is also the report that the locals have nearly decimated it.

Archie Carr's revised edition of his sea-turtle saga has an epilogue bringing the tale up to date, including information on worldwide turtle migrations And mxploitation. Carr ruefully acknowledges that his original dream of farming sea turtles in order to protect wild stocks may have succeeded all too well. Farms were, in fact, set up after he promoted the idea, but the resulting demand for turtles increased even more than the new supply.

Yet at the same time, he notes, more and more countries have joined the conservation forces. Beaches have been declared off-limits to turtle hunters, and artificially established nesting colonies have taken hold.

Scientists haven't yet been able to answer several crucial questions about the animals. Among them: How are they able to accomplish their global navigation? And how do they find nesting sites?

As work continues to try to answer such questions, scientists are finding the response heartening to one request for help. To track migration patterns, they have placed tags on many turtles. People who find the tagged turtles are asked to contact the researchers, and so far the result has been gratifying, Carr notes.

Throughout this new edition of ''So Excellent A Fishe,'' the author's readable style and his relish for sea turtle research are as refreshing and stimul!ting as they were when the first edition appeared.

Mary S. Cowen is a free-lance writer living in Concord, Mass.