Description: English: Alder J. & Hancock A. (1845-1855). A monograph of the British nudibranchiate Mollusca: with figures of all the species. The Ray Society, London. Published in 8 parts:Part 7 Fam.1 Plate 21a [1855] Thecacera pennigera (Montagu, 1815). Date: 1855. Source:
https://www.biodiversitylibrary.org/item/131598#page/194/mode/1up. Author: Albany Hancock.