I have changed your topic from "The Hound of the Baskervilles" to "The Adventure of the Speckled Band." I don't recollect that there was any reference to "The Adventure of the Speckled Band" in Arthur Conan Doyle's "The Hound of the Baskervilles," although Watson frequently refers to other cases in his stories and I may have forgotten seeing such a reference. The speckled band in "The Adventure of the Speckled Band" is a poisonous snake called a swamp adder which the villain Dr. Grimesby Roylott used to kill one of his stepdaughters and is currently using in an attempt to kill his other stepdaughter, Helen Stoner. The reader does not find out what the "speckled band" really is until the very end, because no one had seen it except Dr. Roylott and Julia Stoner, the girl he murdered two years earlier. When Julia was dying from the snake bite she told her sister:
‘Oh, my God! Helen! It was the band! The speckled band!’
Julia dies before explaining further, and Helen never understood what Julia meant. Dr. Roylott gets justly punished for his fiendish machinations against his two stepdaughters, intended in both cases to prevent them from marrying and claiming the portions of their deceased mother's estate due to them. The snake is driven back into Roylott's room and attacks its owner. Holmes and Watson find him dead, with "his eyes fixed in a dreadful, rigid stare."
Round his brow he had a peculiar yellow band, with brownish speckles, which seemed to be bound tightly round his head. As we entered he made neither sound nor motion.
“The band! the speckled band!” whispered Holmes.
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