Prehistorical romance: a new genre in popular fiction; The Clan of the Cave Bear, by Jean M. Auel. New York: Bantam. 1980. Paperback. The Valley of Horses, by Jean M. Auel. New York: Crown, 1982, paperback, Bantam. The Horse Goddess, by Morgan Llywelyn. New York: Houghton Mifflin, 1982, New York Pocket Books, 1983. 439 pp. $3.95. Lion of Ireland, by Morgan Llywelyn. New York: Houghton Mifflin, 1979. New York: Berkley Books, 1982. 559 pp. $3.75. The Wicked Day, by Mary Stewart. New York: William Morrow & Co., 1983. 453 pp. $ 15.95. The Crystal Cave, by Mary Stewart. New York: William Morrow & Co., 1970. Paperback. The Hollow Hills, by Mary Stewart. New York: William Morrow & Co., 1973. Paperback. The Last Enchantment, by Mary Stewart. New York: William Morrow & Co., 1979. Paperback.

There is a new genre shaping up on the popular fiction scene. It is science fiction in reverse, going backward in time to lost worlds and civilizations. Pocket Books calls it historical fantasy. I prefer to call it ancient fiction. The books in this genre contain the usual fiction elements with anthropological details added. These contain varying amounts of fantasy. When they include historical figures, those figures are so wrapped in myth and legend it is difficult to know where the truth ends and the myth begins.

Some are solid candidates for the ancient fiction genre. Those include: Jean M. Auel's ''The Clan of the Cave Bear'' and ''The Valley of Horses'' (both currently on the mass market paperback best-seller list), Morgan Llywelyn's ''Lion of Ireland'' and ''The Horse Goddess'' (published earlier this fall by William Morrow & Co. and currently on the fiction best-seller list) and her Merlin Trilogy - ''The Crystal Cave,'' ''The Hollow Hills,'' and ''The Last Enchantment.''

It would be stretching it to call ''The Clan of the Cave Bear'' and ''The Valley of Horses'' historical fiction since they are set in the prehistoric Stone Age. These two books are the first in Miss Auel's ''Earth's Children'' series.

For research she lived with Eskimos and learned their survival skills. Miss Auel's books place more emphasis on the anthropological details that characterize this genre than the other books. At times these details can be overwhelmingly boring, as in her exhaustively detailed account of the making of a hand ax. The plot, however, is always interesting.

In ''Clan of the Cave Bear,'' blond and beautiful Ayla, an orphaned Cro-Magnon girl, is adopted and raised by a tribe of Neanderthals who call themselves the Clan of the Cave Bear. Each member of the clan has a totem, an animal spirit that protects him and shapes his destiny. Ayla learns about plants and the healing arts from Iza, the clan's medicine woman. She learns about the clan's religion, a blend of magic, superstition, and pantheism, from Creb, the clan's holy man.

Ayla teaches herself to hunt. This brings her into conflict with the other clan members, since women are not allowed to hunt. Because of her powerful totem , the spirit of the Cave Lion, the clan men grudgingly allow Ayla to be initiated as a hunter. Still, Ayla is resented and feared by many of the clan members because she is so different from them. By the end of the story, Ayla is forced to leave the clan, and she sets out in search of others like herself.

In ''The Valley of Horses,'' Ayla is all alone in a lush and fertile valley with only a horse for companionship. She perfects the skills she learned from the clan, which include hunting for small animals, toolmaking, and food-gathering. She teaches herself new skills such as firemaking, riding, and hunting large animals. About the only thing she doesn't discover is the wheel. Miss Auel describes each newly acquired skill in minute detail.

There is also romance in ''The Valley of Horses.'' When Jondalar, a Cro-Magnon man as beautiful as Ayla, arrives in the valley, Ayla learns about love and speech, and prepares to leave the valley to join Jondalar's tribe.

Morgan Llywelyn's ''Lion of Ireland'' and ''The Horse Goddess'' are part of a series about the Celtic peoples. A third volume in the series, ''Bard,'' will be published by Houghton Mifflin next fall.

''The Horse Goddess'' bears some striking similarities to the Jean M. Auel books. Set in the 7th century BC, it tells the story of Epona, daughter of a Celtic chief. Epona, like Ayla, is blond and beautiful. She has been born with Druidic powers, above and beyond the inner voice that all Celts have. The fearsome high priest, Kernunnos, also known as the shape-changer for his ability to transform himself into an animal, wants to train Epona for the priesthood.

Celtic men and women are equals - the women learn to fight and can become priests. Scared and repelled by the Druid, Epona refuses to submit to his tutelage. She flees her mountain village with a group of visiting Scythian horsemen. She falls in love with the horseman's leader, Prince Kazhak and joins him in his nomadic life on the steppes where she learns to ride and train the powerful Scythian horses. Kernunnos, determined to bring Epona back to the Celts , transforms himself into a savage wolf and follows her across the steppes.

To ease her way into her strange new life with the Scythians, Epona teaches herself to use her Druidic powers to control animals and the elements. Forced by Kernunnos to leave the Scythians, Epona returns to her tribe. She destroys Kernunnos and becomes the new high priest. Her introduction of the Scythian horse, suitable for riding and pulling chariots, greatly changes Celtic life and the course of European history and will elevate Epona's status among the Celts to that of goddess of the horse.

A historical figure, Brian Boru, is a near-god in Irish poetry and legend. In ''Lion of Ireland,'' set in the 10th century, Brian's story is told, from the fateful night in his childhood when the Northmen sack and burn his village, to the day in 1014 when he is killed in battle by a Norse warrior. Between, Brian grows to manhood, studies in a monastery, learns how to fight, and unites Ireland under his kingship.

Along the way there is plenty of romance and adventure as well as abundant details of Celtic life, as there were in ''The Horse Goddess.'' There is also fantasy: a subplot involves Brian's romantic relationship with Fiona, a Druid. She, like Epona, the horse goddess, can control the elements and the animals. Brian is a Christian, as is most of Celtic Ireland in the 10th century. Yet the ancient religion of the Druids is still practiced. Through Fiona, it has great influence on Brian's life and destiny.

Many other books have been written about the Arthurian period, enough to create a separate genre. Yet Mary Stewart's Arthurian novels are written in such a way that they can be considered ancient fiction. Unlike other Arthurian novels , her Merlin Trilogy begins long before Arthur is born.

The first volume of the trilogy, ''The Crystal Cave,'' begins when Merlin, the narrator of the trilogy, is only six years old. His story and Arthur's continue in ''The Hollow Hills'' and ''The Last Enchantment.'' In her latest novel, ''The Wicked Day,'' the Arthurian story is continued with Mordred, bastard son of Arthur, and the central character. Like Brian Boru, Merlin and Arthur are figures shrouded by the mists of time. Merlin, like Epona, Kernunnos, and Fiona, has Druidic powers with which he can control nature and thus, the destiny of men.

Like the novels of Jean M. Auel and Morgan Llywelyn, the Stewart Arthurian novels are filled with adventure, romance, fantasy, and details of everyday life - in this case, life in 5th-century Britain. It is the combination of these elements that makes ancient fiction exciting and interesting reading.

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