MOLLUSCA OF PLYMOUTH SOUND. 181 



was a mottled rosy brown with yellow spots. This colour has largely 

 persisted in alcohol. The two tentacular appendages on each side 

 were as long as, or longer than, the tentacles proper, as Alder and 

 Hancock have represented them, and they were constantly waving 

 about while the animal was alive. The branchise were ten in number, 

 but the anterior plume was deeply bifurcated as usual, thus forming 

 eleven points. The posterior plume was not bifid. The lateral filaments 

 were four on each side, the anterior ones very small, the posterior 

 pair very broad at their bases and bifid. 



These dorso-lateral rows of filaments are plainly homologous with 

 the ridges found in the genera Polycera, Goniodoris, &c., and these 

 also with the " epipodial folds " (Lankester, after Huxley) of Archi- 

 doris, so that a term to convey this idea would be of service. The 

 " pallial ridges " of Alder and Hancock ought therefore to be termed 

 " epipodial ridges.^' These structures are of considerable morpho- 

 logical importance and exist to a greater or less extent in every 

 British species (at least) of the family Dorididse, though often repre- 

 sented by veiy specialised vestiges (e. g. the branchial appendages of 

 Thecacera) ; and I believe their homologues are to be seen in Tri- 

 tonia and Lomanotus, becoming strangely modified in the Eolididge. 

 Additional ridges are sometimes developed, viz. a median, and a pair 

 of sublateral (cf. Goniodoris, Polycera, Idalia elegans and Leachii). 



In view of the variability of the branchiae and epipodial filaments 

 in this genus I think it must be regarded as certain that Forbes's 

 two specimens of Idalia infequalis are in reality varieties of I. aspersa 

 (cf. Alder and Hancock's Monograph, Appendix, p. v). They were 

 obtained together in thirty-five fathoms in St. Magnus' Bay, Zetland. 

 The habit of associating in couples seems to be very common among 

 Nudibranchs (cf. Giard, p. 502), and I have noticed it in species 

 which are usually found in large groups {e.g. Goniodoris nodosa). 

 If I should prove to be right in regarding the Zetland Idalias as 

 varieties of I. aspersa, the Scarborough Tritonias {lineata, A. and 

 H.) as varieties of T. pleheia, and the various British forms of Loman- 

 otus as all of one species, then we should apparently have three 

 instances of selective association between similar varieties in so low 

 a group as the Mollusca. 



6. Ancula, Loven. 



12. A. CRiSTATA, Alder. 



In March of this year Mr. W. B. Hardy took a fine individual of 

 this species from a tide pool below the ladies' bathing place. It was 

 an inch in length when fully extended. I obtained another specimen 

 between tide-marks near the same place in April. 



