Several days later, the eternal fog still covered the city. I was treating a case of phossie-jaw in a middle-aged match worker. My signature was still wet on the prescription for morphia when the worker suddenly spoke up - quite impossible in a patient whose jaw had entirely dissolved into a sort of calcareous mush - taking me momentarily aback.

"Good morning, Watson."

This time, however, I regained control with a steely resolve. I did not even look up.

"Good day, Holmes. I hope you realize you have just wasted fifteen minutes' valuable consulting time."

"I do apologize, dear fellow. The streets remain dangerous, and I had to see you. It was the excitement of finishing a case that has been puzzling me for some time."

"The Hampstead trombonist decapitations?"

"The very same. I have found what I believe to be an answer, Watson, in forensic palaeontology." Holmes fidgeted in his chair nervously. "Don't you want to know how I feigned a jaw entirely dissolved by phosphor poisoning?"

"I most certainly do not."

"Very well. It is a case, Watson, that will go down in the annals of all cases in which a dinosaur was used as a murder weapon. For there is a human being behind these deeds, Watson, make no mistake of it." Out of a carpet bag on the floor he drew another volume of bound periodicals. "These are the observations of a Mr. Barnum Brown, who has recently discovered a colossal ornithopod in the fastnesses of Alberta which he names Corythosaurus Casuarius. This creature, a member of the hadrosauridae or duck-billed dinosaurs, possesses a singular crest on its skull - a hollow, air-filled bony structure which some palaeontologists have wrongly supposed to have been used for breathing while the animal snorkelled like a scaly submarine. There is, however, a slight drawback to this theory in that the crest possesses no exterior nostril �"

"The murders, Holmes."

"Ah, yes. There is, you see, Watson, a school of thought who believe the crest formed a sort of resonating chamber with which the beast would be capable of making distinct musical notes - not unlike, one would imagine, a trombone."

"Holmes, I cannot see any conceivable reason why a house-sized prehistoric creature would wish to make a noise like a trombone."

"These creatures were not the sharpest of sorts, Watson. Some of them needed secondary brains in their abdominal regions to remain capable of coordinating their movements. It is also true that large creatures do not necessarily have acute senses; the rhinoceros is notoriously short-sighted, and relies on its sharper sense of hearing to detect approaching hunters.

"Consider, then, a herd of such creatures. Like African herbivores, they might not all be members of the same species. Multiple species of African antelope often gather at the same water-hole to drink. However, our antelope weigh five tons and are only marginally cleverer than the pond scum they are lapping up. It is quite possible that they might elect to mate with the wrong species if they are not provided with some constant audible cue. Some species of hadrosaur might therefore have made noises like trombones, as if to say, Here I am! I am a trombone-crested hadrosaur, and other trombone-crested hadrosaurs may profitably choose to mate with me. But the noises they made would not be of interest solely to their mates. They might also have interested their predators."

"Good God, Holmes", I cried. "Are you telling me those poor wretches died simply because their instruments made sounds resembling that creature's natural prey?"

"I not only believe it", said Holmes, "I intend to prove it, by the most direct means possible."

He drew out the trombone from his bag, along with a copy of the sheet music for Thaxted.

***

"You will not need your pistol, Watson. It would be entirely useless against the creature. Shooting it accurately in the brain would be as hard as hitting a bull from a hundred yards, and I imagine even four-five-five ball would simply bounce off its scaly hide."

We were walking through a wilderness of winter-deadened trees, in a fog in which an entire herd of hadrosauridae could have stood shoulder to shoulder unobserved. A bandstand loomed out of the murk. Posters announced a forthcoming event involving royalty.

"There do not seem to be any dinosaurs in the vicinity, Holmes. I feel sure that such large animals would advertise their presence."

"A predator", said Holmes, "never advertises its presence." And he raised the trombone to his lips, and blew out the final bar of Thaxted. A tear rose to my eye at the thought of the country all of whose ways are gentleness, all of whose paths are peace.

Then the hairs at the back of my neck stood on end as I heard a distant, clumsy crashing from far away in the fog, as if a drunken coachman were attempting to drive an omnibus through heavy brush.

"The hunter's afoot", said Holmes. "And it is we who are the game." He motioned to me to follow as he stole away.

"Confound it, Holmes! I hadn't expected your damnable theory to actually be correct!"

"My theories, Watson, are always correct." With that, he plunged into a nearby drain or ha-ha and squelched ponderously along it, not appearing in any hurry to get to the other side. "Into the water, Watson. In this fog the beast will hunt by scent."

I have no shame in relating that I piled into the water more smartly than I have ever piled into water before, particularly since I heard, at a somewhat lesser distance now, the immense crackling whisper of something dreadfully, fearfully heavy walking across the carpet of fallen leaves on the Heath towards our former position. Walking slowly, and appearing to deviate to left and right, like a questing hound. I believe I actually heard breath, escaping like a head of steam from a ship's boiler.

"Do not make a sound, Watson, for your life depends on it."

Holmes claims that he still saw nothing at that point - I would have been able to see nothing even had the beast been standing right next to me, for I had my eyes tight shut. But at that point, we heard another sound, deep and sonorous, singing out through the fog - the sound of the low notes of a violin. It is difficult to convey how I knew purely from the sounds I heard through the mist that a Mesozoic lizard was cocking its head on one side, but somehow I knew this was what was happening, as if a bird on a branch had heard another bird whistle, or a dog heard its owner call.

We heard it plunge away through the fog.

"As I suspected", said Holmes. "There are two hadrosaur species."