654 SIR c. ELIOT ON NUDiBRANCHS [June 19, 



Thoradisa Bergh. 

 The following species have been referred to this genus : — 

 1. Th. villosa (A. & H.). 



2. Th. Tnaculigera Bergh. 



3. Th. stellata Miot. 



4. Th. tristis Bergh, 



5. Th. hilaris Bergh. 



6. Th. ? maculosa Bergh. 



7. Th. ? carinata Bergh. 



8. Th. clandestina Bergh. 



9. Th. ladislavii (von Jher.). 



10. Th. ? miUegra7ia (A. & H.). 



11. Th. crosslandi Eliot. 



12. Th. ? duhia Bergh. 



13. Th. ? pallida Bergh. 



14. .77i. ? catidata 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 Disco- 

 doridibus, dentes extimi denticulati. Penis inermis." In his 

 most recent work (The Opisthobranchs of the ' Siboga ' Expedi- 

 tion, 1905) he returns to this __ definition and says: — "Die 

 Gattung .... zeichnet sich im Ausseren besonders durch die 

 villose Beschaffenheit des Rlickens aus. Die ausseren pleuralen 

 Zahnplatten sind sehr schmachtig und kammfdrmig 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, bu.t 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 Staurodoris 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. 



