Ten minutes later I was staring at Homes as though he were the starkest madman.

"A megalosaurus?"

"The very same. You know my maxim - when you have eliminated the impossible, whatever remains, however improbable, must be, etcetera, etcetera."

I began to feel somewhat light-headed. "Holmes, the megalosaurus has been extinct for nearly two hundred million years."

"Not so, Watson." At this point Holmes fished out a stack of books and papers he had been concealing in his trombone case and began flicking frantically through a periodical. "I considered all the animals that currently exist in the world and are capable of killing a man, and eliminated them one by one from my inquiries. The leopard lacked the sheer height to attack from above unless concealed in a tree, and I could find no claw marks in the bark of any tree hereabouts. The elephant, meanwhile, whilst possessing sufficient elevation, would gore or trample in preference to biting. Finally, I concluded that the attacker was an animal unrecognized by conventional zoology.

Therefore I switched from zoology to palaeontology. The characteristic dual puncture wounds of a sabre-toothed tiger attack were not present, nor were the sixfold blunt contusions that would indicate a charging uintatherium. I went through ten heavy tomes at the British Museum before I alighted on my culprit. A megalosaurus had performed this crime, Watson - a huge Jurassic beast moving on powerful three-clawed feet and with a mouth like a cavern of stalactites. I know what you will say, Watson - nowhere on Earth does a megalosaurus currently exist. But there are reports from the Belgian Congo of creatures that resemble a stegosaurus more than a stegosaurus's maiden aunt, and in addition, there are the remarkable accounts which I now commend to you." He had now found the pages he had been looking for, and set to reading aloud enthusiastically.

"Proceedings of the London Zoological Institute, June 13th, 1917: an address to be delivered by Professor Challenger to a general assembly of all members, regarding extraordinary discoveries recently made in a high tributary of the Amazon�"

"That was all poppycock. Totally discredited. The expedition brought back no evidence of the existence of living dinosauria in the Amazon whatsover."

"You must remember, Watson, that the generally accepted public account of the expedition is that of Mr. Edward Malone of the Daily Gazette", admonished Holmes. "But I have recently been across the Channel to purchase Professor Challenger's own version of events and it makes most enlightening reading. As you know, the Professor's account was savaged by Her Majesty's censors, only a few select copies making their way here across the Channel after having had to be published in France."

He switched to a different volume, this one hardbound and bearing a lurid cover engraving of a lady in night attire being improbably menaced by a tumescent Mesozoic reptile.

"September 13th, 1916. Violently, bestially drunk on that fermented mash of giant spiders the natives call ghula-ghula. Still see huge green dinosaurs everywhere. Am having to gradually force myself to face the horrid truth that they might actually be real. After tea, with Roxton, Malone and Summerlee out of the camp, disported myself with two of the hallucinatory Stone Age women hereabouts, enjoying to the full their supple and surprisingly real-seeming honey-coloured bodies -"

"That is quite enough, Holmes", I snapped. "The ravings of a man in an arachnolysin-induced delirium are hardly proof of the continued existence of dinosaurs."

"Agreed, Watson. But if we read on! If we but read on! December 1, 1916. Our return to England has been safely negotiated without either my confederates or Customs & Excise suspecting I had more in my baggage than declared. The egg is ovoid - as is only to be expected - about the size of a large coconut, porous skinned, and a bright saffron yellow in colour. I have been incubating it by stealing into the cargo hold and covering it with rotting kitchen waste, and in addition, whenever possible, sit on the egg personally. When questioned why I was sitting in rotting kitchen waste in the cargo hold by the ship's purser, I simply replied that I was incubating an egg which, when it hatched out, would develop into a twenty foot long maneating lizard, whereupon he simply grinned, tapped his cap in a friendly way, and left me to my own devices.

"February 2, 1917. My hatchling has arrived. Have been feeding it turkey giblets, which it consumes with gusto. Have named it Gladys, after Malone's carnivorous reptilian fianc�e. Gladys very attracted to bright objects and movement. Already appear to have lost the cat, a huge fluffy useless article good only for destroying valuable furnishings. Wife distraught. Am beginning to like Gladys already.

"April 10. Gladys over six feet long now. Intend to take her in to one of that confounded bore Walton's lectures on the malleability of germ plasm. I will say nothing and announce nothing - just take her in on the end of a long piece of string, and sit at the back, and stare. Then let that pieheaded buffoon deny the existence of living palaeozoons.

Green, the children's music teacher, has packed his things and left. Complained about having to deliver lessons in the same room as Gladys; claims music drives her into a bestial fury. Good riddance, for his music drives me into just such a fury, as he practises nothing but Holst's Jupiter at every hour of the day and night.

"To the point, Holmes."

"September 23. Have negotiated a regular meat supply for Gladys from a Mr. Glass, a heavily bearded importer of fine Irish beef. Have surveyed Mr. Glass's warehouse and suspect his beef was formerly in the habit of whinnying, but his prices are cheap. Two of his associates delivered the first consignment to our drawing room, where Gladys is currently residing. The impertinent lackeys passed doubt on the ability of the window bars to prevent Gladys from escaping. Was only too ready to demonstrate to them how the windows could only be opened on the outside.

"September 25. Donned diver's helmet and steel gauntlets and took half horse carcass in to Gladys as a treat. Disaster! Window open, drawing room empty, three-toed clawprints disappearing over the lawn into the distance."

Holmes snapped the book shut.

"Are you seriously suggesting that a palaeozoic carnivore is stalking Hampstead Heath?"

"I have been checking public records,Watson. There have been over fifteen cases of unexplained decapitations and disappearing animals on and around the Heath in the last twelve months. The police no doubt failed to connect them with this investigation due to the fact that none of them involved trombones."

"So a prehistoric monster is responsible for these killings?"

"Not entirely, Watson. There are still facets of the affair that elude me. For example, in all six cases, listeners reported that the trombonist's solo was accompanied by a lone fiddle part which joined the refrain just before the notes suddenly, tragically choked off. No, my friend - I am thinking that the mind behind these enormities evolved quite recently."