To William Thompson   [1 March 1849]

My dear Thompson

I sh^d. have answered your note by return of Post, but I was in bed all the day before yesterday & was incapable of doing anything yesterday morning. The enclosed diagram will show difference between Chthamalus & Balanus.—¹ You will with great difficulty recognize the genus externally in the British species: internally (when cleaned) there is a conspicuous difference.— I send some good British specimens & a N. American species, in Blotting Paper which shows the characteristic difference much better— Look with lens at inside of the cleaned Brit. spec. & compare with a cleaned Balanus & look at diagram & you will understand difference.— It is curious that the British B. punctatus & Chthamalus are so alike externally, as they belong to different sub-families.— The British Chthamalus has never been named as a Chthamalus; I suspect it is the B. punctatus of Montague; but the nomenclature of the sessile Cirripedes is all guess-work.— As I thought it w^d be satisfactory to you, I have looked through your whole collection & cannot see any Chthamalus; I suspect cause is that you have collected generally those on shells & wood, crabs &c. & not so often on rock.— (NB The walls in Chthamalus are not cellular or tubular as is always case with Balanus)— I have been delighted at going over your collection again to see what an admirable one it is: it will be invaluably useful to me.—²

I have gone through $\frac{4}{5}$ of your Book³ & found much to interest me— The accounts of the Hawks & Corvidæ,—in short wherever you treat much of habits, will, interest, I sh^d. think, everyone.— The discussions on the rarer species being deservedly Irish, will tend to make the Book heavy for the general reader; but I know that you intended your work not as one of amusement, but of instruction.— Why do you & others separate English & Latin Index.— does the English hurt the Latin? Many a time in such cases has something very like an oath come very near my lips, when I have found that I have been looking in a hurry in the wrong index.—

Have you seen Rev. E. S. Dixon (not Dickson) work on Poultry;⁴ it is very good & amusing.

I quite forget whether I told you, that I have quite lately made up my mind to go before the end of this month to Malvern for two months to see if there be any truth in the water cure.— It will lose me two months but if I can partly regain my health, it will indeed answer in every respect, for I am sure I have not worked more than 1 out of the 3 last months. I fear we shall not meet this Spring, without you remain till June in London.—

I am tired so no more— If you are in doubt about any Irish Chthamalus, pray send it me.

Your’s most sincerely | C. Darwin

What labour your Book must have cost you!

Horizontal section through upper part of shell.

you will see that carina & second latera are the same in Balanus & Chthamalus— whereas Rostrum & 1^st latera materially differ.— The Rostrum & carina have a similar structure in Chthamalus.—