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Show 654 SIR C. ELIOT ON NUDIBRANCIIS [ J une 19, T h o r ad isa Bergh. The following species have been referred to this genus:- 1. Th. villosa (A. <fe H.). - 2. Th. maculigera Bergh. 3. Th. stellata Eliot. 4. Th. tristis Bergh. 5. Th. hilaris Bergh. 6. Th. ? maculosa Bergh. 7. Th. ? carinata Bergh. 8. Th. clandestiita Bergh. 9. Th. ladislavii (von Jher.). 10. Th. ? millegrana (A. &■ H.). 11. Th. crosslandi Eliot. 12. Th. ? dubia Bergh. 13. Th. ? pallida Bergh. 14. Th. ? caudata Farran. The queries are due to the authors who have created the species or referred them to this genus. Bergh's original diagnosis of the genus (Semper's Reisen, Heft xii. 1877) is:-" Forma corporis fere ut in Discodoridibus, dorso fere villoso. Armatura labialis nulla. Lingua ut in Discodoridibus, dentes extimi denticulati. Penis inermis." In his most recent work (The Opisthobranchs of the ‘ Siboga ' Expedition, 1905) he returns to this definition and says:-" Die, Gattung . . . . zeichnet sich im Ausseren besonders durch die villose Beschaffenheit des Riickens aus. Die ausseren pleuralen Zahnplatten sind sehr schmachtig und kammformig entwickelt." Meantime, however, several forms have been referred to the genus which have neither a villous back nor pectiniform external teeth. The points emphasised by Bergh in the passages quoted above seem to characterise the genus with fair distinctness. The animals are externally not unlike Discodoris, but the dorsal surface, instead of being granulate or tuberculate, bears soft elongated processes of moderate length. A labial armature is absent, at least in the typical forms. The rhachis of the radula is bare and the side teeth are simply hamate, but towards the end of the row they become thin and have a tendency to split up into hair-like denticles. If, however, these denticles are not developed, I do not think that their absence is sufficient to exclude a form from the genus. It is admitted that in Platydoris, Halgerda, and Staurodoi'is such denticles may be present or absent, and they should not be used too rigidly as a generic character. On the other hand, it may be justly said that in such genera as Thordisa, Platydoris, and Halgerda the outermost teeth show a strong tendency to split and develop denticles; whereas in Archidoris, Discodoris, and other genera they have a tendency to diminish in size but to remain entire. |