of the Nudihranchiate Mollusca. 39 



inhabiting the surface of the earth. Indeed, the fact that many 

 richly-coloured animals habitually lurk under stones and in ob- 

 scurity would lead us to the conclusion that light has no influ- 

 ence in evolving these gorgeous tints. Be that as it may, how- 

 ever, colour, in the creatures which we are considering, is ex- 

 tremely capricious ; and experience has shown that it is impossible 

 to be sure, a priori, whether a certain colour is a permanent spe- 

 cific mark or otherwise. Eolis Landsburgii always possesses its 

 characteristic amethystine tint, E, rufibranchialis its brilliant 

 scarlet, and I have never met with an E. coronata which did not 

 possess a delicate dash of ultramarine in the papillse. But other 

 species are far more variable, such as Doris tuberculata, which 

 boasts of all the colours of the rainbow, D. pilosa, Pohjcera 

 quadrilineata, &c., which vary considerably in the intensity of 

 their markings ; while of a great many other species some indi- 

 viduals are dark, or highly coloured, and others of the palest : 

 such are Polycera ocellata, Eolis papulosa, Embletonia pulchra, 

 Hermcea dendritica, &c. The danger, however, of naming these 

 animals from the colour, especially when they are imperfectly 

 known, is best illustrated by such examples as Doris pilosa, 

 which has burdened science with not a few synonyms, having 

 been called by Loven, Doris fusca — by Leach, Doris ochracea — 

 by Fleming, D. niffricans ; whereas the species was in each case 

 identical, and simply varied in colour. Doris bilamellata also 

 received the name o^ D. fusca from Miiller, but I have repeatedly 

 met with it of the palest tint. Tritonia Hombefrgii is found 

 sometimes purple, sometimes yellow, and Macgillivray described 

 a specimen as a new species, under the name of T. atrofusca : 

 it was, however, only a dark variety of T. Hombergii; and I 

 have lately met with it of a pure white. Even the careful authors 

 of the beautiful Ray Monograph have not steered quite clear of 

 this error, for I have found specimens of Eulis aurantiaca in 

 which no trace of orange colour was evident ; and in the 'Ann. 

 Nat. Hist.^ for 1843 a species was described under the name of 

 E. pallida, from its want of colour, which it behoved its dis- 

 coverers to re-name, at a subsequent period, as E. picta, owing 

 to the rich and variegated tints with which most examples were 

 adorned. Both pale and dark varieties occur on the shores of 

 the Mersey. It would appear, therefore, that no degree of cer- 

 tainty attaches to the fixity of colour as a specific character, and 

 it is only when the species has been well observed that any 

 reliance can be placed upon the uniformity of its tints, and this 

 at too late a period for it to be of any real service in nomencla- 

 ture. Not, however, that this danger is peculiar to the naming 

 of the Nudibranchiata ; it is a generally besetting one ; and num- 

 berless examples might be adduced, both from the animal and 



