Many writers, like comedians, have a shtick, a running joke that comes up now and again, inviting the reader to smile. For Richard Fortey, the British Museum paleontologist who wrote Trilobite! Eyewitness to Evolution, it is the passengers on the 8:02 from Henley-on-Thames who accompany him into work in London each morning. They are amazed, amused, and bewildered by the stories he tells them about his job; they even ask, at one point "What do you actually do?"

Fortey acknowledges the commuters for their help in writing this book. In fact, Trilobite! would be an excellent choice for reading on the train. Although overwritten at times, the book bounces around geological eras from the Precambrian right up to today in a lively fashion. It bounces around the globe, too, from Spitsbergen to the Antarctic. It is easy to imagine sitting next to Fortey as he regales you with tales of doughty fossil hunters braving the elements in Wales, a tragic German Jewish scientist caught up in the Holocaust, and evolutionary biologists catfighting over the fine points of Darwinism. His storytelling skills make this book a good read - but unfortunately, not a satisfying book about trilobites.

This is especially lamentable because there is no book about trilobites aimed at the nonspecialist audience. Considering that these arthropods - their closest living relatives are horseshoe crabs - dominated Earth's seas for millions of years, it is surprising that, before Fortey, there were only two books in English devoted to them. Both are addressed to paleontologists, and the texts are littered with specialist terms.

Fortey begins Trilobite! with a typical passage of bravura prose: he is on the Cornish coast, walking along a cliff above the pounding surf, retracing the steps of a character in a Thomas Hardy novel. The hero, clinging to the slate after a fall, comes face to face with the stony eyes of a fossil trilobite. This does seem to be the first mention of trilobites in English literature, and the author uses it as an attractive hook on which to hang an introduction to geology and stratigraphy - but it is also rather disappointing. A Pair of Blue Eyes is not a Hardy book many readers will know, and there are, in fact, no trilobite fossils to be found in this part of Cornwall.

The following three chapters - "Shells," "Legs," and "Crystal Eyes" - are the closest that Trilobite! comes to straightforward exposition. Fortey describes these parts of the fossil animals, and provides some information on just how trilobites must have lived, eaten, and been eaten. The chapter on their eyes - which are nicely preserved in fossils because the lens was part of the hard shell - is the best, offering a fascinating explanation of different styles of vision and the reasons each may have evolved. It contrasts with the lengthy description of trilobite shells. Fortey provides a detailed enumeration of the various parts of the trilobite shell, from cephalon (head) to pygidium (tail), but tells us very little about how they lived.

In part, this is because we truly know less about trilobites than dinosaurs, which lived millions of years later and are more closely related to modern animals.

But this section also reflects Fortey's own professional bias: he has spent his career collecting fossils, giving them names, and assigning them to the correct drawers in the British Museum. He is more interested in fossils, and those who collect them, than in the extinct animals they represent. A better title for the book would have been Paleontologist!

Following these chapters, Trilobite! goes off in several directions that do not hang together, even though each has an obvious connection to trilobites. Many pages are devoted to Stephen Jay Gould, perhaps the most prominent and controversial American biologist alive today. Fortey knows the man, and has intelligent comments to make about the rise and fall of the Burgess Shale fauna and the punctuated equilibrium controversy, but it is not clear why he includes this discussion in this book. Perhaps the back-and-forth between Gould and paleontologist Simon Conway Morris demonstrates how scientific arguments are carried out in print and in public, but these pages cast little light on trilobites.

The story of Rudolph Kauffmann, a Jewish scientist killed by the Nazis, is also tangential to the book's subject. The chapter devoted to paleomagnetism and plate tectonics makes more sense: the discovery of the history of the planet's magnetic field and its reversals, and of continental drift, helped to clear up many geological mysteries. Since trilobites dominate the fossil record for such long periods, their remains helped unravel both of these questions. And once the peregrinations of the ancient continents were understood, it was easier for paleontologists to sort out which organisms lived where and when. Unfortunately, Trilobite! has few footnotes and no bibliography, so readers interested in finding out more about these topics will have to locate the many books and articles on their own.

All in all, Trilobite! works if you want to be swept up by Fortey's enthusiasm. It gives an excellent picture of what a scientist does, from cracking rocks with a hammer to choosing names for new (old) species; from listening to violent disagreements at scientific meetings to eating steamed horseshoe crab. Many readers will enjoy Fortey's collection of anecdotes and personal encounters with other scientists. And the book offers an entertaining introduction to some of the issues that have moved biology and geology in the last 30 years. But if you are hoping for a systematic introduction to this interesting group of animals, Trilobite! falls short. There are so many good books on dinosaurs, dealing with the biology and ecology of all the major groups and with even some individual species, but there is still nothing of the sort for trilobites.