678 SIR c, ELIOT ON NUDiBRANCHS [June 19, 



regai'ded as more primitive than the forms which have developed 

 bi'aiichife and lateral lamellae, or as retrograde. Perhaps the latter 

 hypothesis is more probable. 



To the five species mentioned in Bergh's ' System ' may be 

 added ; — 



6. PI. ivalteri Krause. 



7. PI. picteti Andre. 



8. PL alba Eliot. 



9. PI. pallida Bergh. 

 10. PL suluensis Bei-gh. 



The family, especially the genus Pleurophyllidia, is probably 

 cosmopolitan, being at present recorded from Spitzbergen, the 

 Northern Atlantic, the Mediterranean, Brazil, California, "Val- 

 paraiso, Honolulu, Japan, China, the Peisian Gulf, India, Ceylon, 

 the Malay Archipelago, East Africa, and West Australia. 

 Linguella seems confined to the Indo-Pacific, all the eight species 

 being recorded from Indian, Chinese, and Japanese waters. 

 Pleuroletora has much the same distribution, with the striking 

 exception of P. walteri from Spitzbergen. 



The coloration is usually sober, but some of the tropical species 

 are rose-coloured, and Pleurophyllidia iceniolata is striped with 

 purple and yellow. 



In all the genera each species has a characteristic dentition, but 

 it may be observed that in many species only a single individual 

 or very few have been examined, so that the radul^e may show a 

 wider range of variation than is at present known. The arrange- 

 ment of the teeth is in all cases essentially the same. There is a 

 rhachidian tooth bilaterally symmetrical and bearing sevei'al cusps, 

 a somewhat clumsy first lateral which sometimes resembles half 

 the rhachidian tooth, and a varying number (from 3 in Pleuro- 

 leura picteti to 180 in Pleurophyllidia formosa) of subsequent 

 hamate laterals. The species difler chiefly in the amount of 

 denticulation, and all degrees of it are found, from Pleurophyllidia 

 natalensis and P. cygnea, in which all the teeth are smooth, to 

 Ple%irop)hyllidia undidata^ in which they are all denticulate. The 

 various transitional stages can be easily traced, but it is hard to 

 say whether we should regard them as the gradual disappearance 

 of denticles fi-om denticulate teeth or the gradual splitting up and 

 serrulation of smooth teeth. It may be noticed, however, that 

 the radula of such forms as P. cygnea is very like that of Tritonia, 

 and the jaws are similar in the two groups. The buccal parts 

 might therefore be regarded as derived from those of Tritonia ; 

 but otherwise the Pleurophyllidiidse occupy an isolated position, 

 and their pecuHar chaia,cteristics are probably correlated with 

 burrowing habits, which are known to prevail in some species. 

 It would be interesting to know if their resemblance to the 

 Phyllidiidse, which is merely external, can be explained by any 

 similarity in their manner of life. 



Of the Newcastle specimens, Pleurophyllidia formosa and 



