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 (a synonym of Pseudancistrus barbatus, the bearded catfish) (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 peroni (‘peronii’ was an incorrect subsequent spelling) 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.

Monacanthus 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.