MOLLUSCA OF PLYMOUTH. 433 



of a species of ^olid evolved by the selection of the most troglo- 

 dytesAWe individuals of each generation. Since, however, it is only 

 at Wimereux that the relations noticed by Giard to exist between 

 the two animals are known even to approach constancy, and as simi- 

 larly coloured specimens occur abundantly elsewhere, this view can 

 be entertained at present only doubtfully. But as local colour- 

 varieties of this species are not rare, the question could be settled 

 almost conclusively if naturalists on the different coasts would notice 

 whether the local race has or has not the special relations of mimicry 

 to some locally abundant Actinian upon which it feeds or near 

 which it lives. The colours of the local races ought to vary with 

 the colours of such locally abundant species of Actinian. It they 

 should be found to do so, then a strong case for true mimicry could 

 be made out ; if not, then selection of the individuals most resembling 

 the locally abundant Actinian cannot have taken place, and the 

 species, like its varieties, must find its causal explanation elsewhere. 



18. tEglidiella, Bergh. 



22. ^OLIDIELLA AlDERI, Gocks. 



This rare species, discovered at Falmouth in 1848 by Mr. W. P. 

 Cocks, and not recorded from any other part of the English coast 

 (Mr. Cocks himself never saw it after 1849), was found in large 

 numbers by Mr. Bourne and Dr. Fowler on the shores of the Yealm 

 estuary between Fox Cove and Thorn Cove late in October, 1889, at 

 low water ; and has been taken on subsequent occasions, though in 

 less abundance, from the same locality. The ground here is fairly 

 firm, consisting chiefly of small stones with an admixture of coarse, 

 slightly muddy sand, while old oyster-shells are scattered about. 

 Most of the specimens have been found under these shells or under 

 stones. A yellowish-white species of Actinian, Sagartia (sp. incert.). 

 is very common here also, being found attached to the pebbles or 

 even free, but generally in either case half buried in the sand, and 

 shrinking into it at the slightest touch. It is very probable that 

 this anemone constitutes a chief source of the -Solid's food. Curi- 

 ously enough, the ^olid and the Actinian are very like each other 

 in colour and form, and it was easy when collecting the former to 

 mistake at times half-buried specimens of the >S'ogfarim f or the Nudi- 

 branch. This case maybe, like that of JEolis papillosa Sbnd Sagartia 

 troglodytes, one of true mimicry, and I am collecting facts bearing 

 upon the matter. 



Early in February this year, upon another visit to this locality, I 

 could find only a few specimens of the -^olid, but whether the 



