Darwin's Fireplace and the Baker's Uncle
[left, vertically stretched]: Photo of the fireplace in Charles Darwin's study
[center, vertically stretched]: Alfred Parsons' depiction (1882) of Charles Darwin's study in Down
[right]: an illustration (1876) by Henry Holiday to Lewis Carroll's The Hunting of the Snark
I think, that Alfred Parsons went beyond just depicting Charles Darwin's study. Especially you won't find the "mouth" in the center image (note (b)) in the photo (note (a)). But as for position and location, that "mouth" matches well with the "Uncle's" mouth (note (c)).
Parsons must have added that arched line (note (b)) deliberately to his painting. Perhaps he had discovered Henry Holiday's puzzles in the Snark illustrations and then played Holiday's game in his own drawing. Is this a kind of insider joke between artists?