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Darwin Correspondence Project

From Albert Günther   [c. 23 March 1868?]1

In Monacanthus penicilligerus 2 & others no sexual difference, at least not on the side of the tail; in some the sides of the tail exactly alike, in others somewhat rougher in the male.

In Monacanthus peronii female smooth, male with a tooth-brush3

In Monocanthus scopas female with a brush of setiform spines (14 inch), male with a cluster of long stiff straight spines 138 long in an example 6 inches long)4


The date is conjectured by the relationship between this letter and a note dated 23 March [1868] in which CD mentions that Günther had shown him male and female specimens of Plecostomus barbatus (DAR 82: B13; see Descent 2: 10–12). In Descent 2: 12, CD noted that Günther had shown him Monacanthus scopas, and referred to it as ‘a nearly analogous case’ to P. barbatus. It is likely that Günther showed CD these fishes at around the same time.
Monacanthus penicilligerus is now Chaetodermis penicilligerus (the prickly leatherjacket).
Monacanthus peronii is now Pseudomonacanthus peroni (the pot-bellied leatherjacket). Males have a row of two or three enlarged scale spines on the lower half of the caudal peduncle.
Monocanthus scopas is now Amanses scopas (the broom filefish). Males have numerous long spines in front of the caudal peduncle, while females have a toothbrush-like mass of setae. In Descent 2: 12, CD cited Günther for information on M. scopas and M. peronii.


Descent: The descent of man, and selection in relation to sex. By Charles Darwin. 2 vols. London: John Murray. 1871.


Note on sexual differences in Monacanthus.

Letter details

Letter no.
Albrecht Carl Ludwig Gotthilf (Albert) Günther
Charles Robert Darwin
Sent from
Source of text
DAR 82.2: 25
Physical description
inc? †

Please cite as

Darwin Correspondence Project, “Letter no. 6531,” accessed on 22 October 2020,

Also published in The Correspondence of Charles Darwin, vol. 16