My niece screams in ecstatic terror when I catch her on the far side of the willow. Rather than eat her, I swing her onto my shoulders. Ethan has unhappily waited for us to rejoin him. He rolls his eyes when Alesha asks me, "Why didn't you go where they have dinosaurs?"

I set her on her feet next to him. "Because I couldn't."

"Nobody can," Ethan tells her. "They can only go back to the Sillian — Sil—"

"Silurian," I say, gently.

"The Silurian Period," he says, enunciating as if his life depended on it. Sih Loo Rih An.

"Sillian." Alesha giggles, says it again, giggles some more. Ethan rolls his eyes again. "Ethan read about it. He said it was boring."

He shoots her a look that somehow fails to incinerate her on the spot. I think. Well, spending a year in the Silurian is a lot like pulling a tour of duty in Greenland, only not as dangerous. The vistas are utterly different but about equally stark. The sense of isolation and the tedium are the same. I sat out my Silurian sojourn at a weather station, contributing my vital bit to paleoclimatology. I had wanted to do it and am not sorry I did it, but, frankly, tropical depressions haven't changed in 400 million years.

I say, "Bor-ring is what I thought until the night of the giant sea scorpions."

Their eyes widen, then Ethan's skepticism kicks in, and the question in his mind is plain on his face: How giant ...?

"Would you like to hear the story?"

He nods. She considers for a moment. "Is it scary?"

"Maybe as scary as Wizard of Oz. Remember how scary the witch was? Think you can handle anything that scary?"

"Is there a witch in this story?"

"No, but there're big monsters," and I lead them under the willow to a bare patch of earth, and with a stick I draw an elongate teardrop in the dirt, add lines dividing the teardrop into segments, add a pair of sweptback, oar-like appendages and then (having saved the best for last) two arms ending in pincers. "That's what these monsters look like. They live in— lived in the water, and some of them grew as big as me." I can tell from Ethan's expression that he's disappointed. "Or even bigger." How big does a critter have to be to be impressive?

Alesha says, "Do they eat people?"

"They eat anything they can get into their mouths. Tell you what, why don't I show you what I brought you instead, and you can go show Mommy and Daddy, okay?"

"Okay! Show me!"

We sit, me with my back against the willow, each of them by one of my outstretched feet. I put my hand into the left pocket of my jacket and say, "Once upon a time, long before there were people or dinosaurs, most of what is now land was at the bottom of the sea. A few rocky islands poked up out of the water, that was all. Hardly anything lived on land except right at the water's edge. Ah, but in the water—that was different. There were strange-looking fish that wore bony armor, and tiny, tiny sea creatures that formed reefs as big as mountains. There was a tribe of animals called trilobites. Can you say that?"

"Trilobites," Alesha says, perfectly.

"That's very good. I'm glad you can say it, so you can tell everybody what this is," and from my pocket I draw a disk of clear plastic. It just about fills the palm of my hand. Embedded in it are three dainty glistening blue-gray trilobites, trilobite exoskeletons, anyway, cast off during molt. They really are quite lovely. I wasn't authorized to collect any specimens for transport to the 21st century, but a marine biologist I knew was good enough to pick these out of the seabed litter for me. It's extremely gratifying to see Alice's eyes and mouth imitate the letter O as I let the disk slide out of my hand, into hers.

Ethan cranes forward to see. "Are those fossils?"

"Nope. They were fresh when they went into the plastic. Little Bit, don't I get a thank-you hug?" I do, and a thank-you kiss on the cheek as well. "Now why don't you go show Mommy and Daddy?"

"Okay," and off she rockets, clutching her piece of the Paleozoic Era.

I settle back against the tree. Ethan squirms impatiently. The sooner I get on with my story, he knows, the sooner I'll be reaching into the right pocket of my jacket.

I begin.