Deep in a mangrove swamp near the Gulf of Mexico, archaeologists have discovered traces of another ancient Mexican civilization.

So far, the only clues to these mysterious people are a four-ton stele, or stone slab, covered with hieroglyphic writing and a 19-inch-high clay figure of a fertility-rite priest."But," says Fernando Winfield Capitaine, director of the Xalapa Museum of Anthropology, "the finds are so promising that the museum is planning to mount an expedition to that area of Veracruz state in cooperation with the National Geographic Society."

The stele was found in November 1986. It is covered with inscriptions in an unknown language but with some Mayan numerals.

The priest in the clay figure is wearing the human skin of a flayed sacrificial victim.

Both are about 1,800 years old.

"That would place them in the second century A.D., which is after the archaeological Olmecs, who flourished in the area from around 1050 to 500 B.C., and before the Mayas who emerged around the fourth century A.D," Winfield said in an interview.

Adding to the intrigue, the Mayas lived in the Yucatan Peninsula, more than 700 miles to the east, Winfield said at the museum in Xalapa, where the stele and figure are kept.

"But there are two dates on the stele in Mayan writing, in their so-called Long Count Calendar made up of dots and bars, equivalent to May 22, 143 (A.D.), and July 13, 156 A.D.," Winfield said.

If the mysterious people did exist, Winfield said, they also appeared to be unrelated to the other two of the four known civilizations that inhabited the gulf coast: the Totonacas, who emerged around the seventh century A.D. in the area where the artifacts were found, or the Huaxtecas farther north, about one century later.

"They could be the Xicalancas, of which Clavijero briefly talked about," Winfield said, referring to the Rev. Francisco Javier Francisco Javier Clavijero (1731-1787), a Jesuit priest who was the first to systematically study Mexico's ancient languages and peoples.

In his "History of Ancient Mexico," Clavijero in three lines briefly mentioned the Xicalanca civilization in relation to the Olmecs, but did not go into details.

In "nahuatl," the trading language of the Aztecs, who came much later, Xicalanca means "the people who live by the Great Body of Salt Water," as the Gulf of Mexico was called.

The two artifacts were found within 130 feet of each other in the village of La Mojarra, where the Piojo River meets the Acula River. The crocodile-infested region is so swampy that it can be reached only by boat, Winfield said.

La Mojarra, with a population of fewer than 200, is up the Acula about 20 miles south of the port of Alvarado in Veracruz state. But neither the village nor the Piojo River can be found on any map of Mexico.

Carved in relief on the previously polished stele, which is of basalt, one of the world's hardest rocks, is the full length profile of a regal figure, a man about 25 years old, holding out in both hands a vessel in a gesture of offering.

"He was probably a ruler, or a chief warrior, or a high priest - or maybe all three since that was the custom in the gulf region in ancient times," Winfield said.

The figure wears ear-stops, a tall headdress adorned with four representations of something like a rain god, trimmed with eight feathers, and a breastplate full of other allegorical figures over what appears to be a pleated cotton shirt or tunic.

A total of 577 glyphs, or picture-words, divided into 21 neat rows running from top to bottom, plus 33 "elements" like the rain-god representations, cover the rest of the slab.