MOLL use A OF PLYMOUTH SOUND. 189 



a quarter inclies long and Thomson's specimens were upwards of 

 one and three quarters inches in length. Alder's marmoratus from 

 Torbay was rather more than half an inch, as also was our largest 

 specimen. Our two other specimens were just half an inch, and 

 Aider's JIavidus from Lamlash Bay was a quarter of an inch in length. 

 Thus on the theory that they are all of one species we find that the 

 oldest examples (as judged by size) have the greatest differentiation 

 of the margins of the sheaths and of the epipodial ridges, that those 

 nearest together in age are closely alike, and that the youngest indi- 

 vidual (fiavidiis) has the simplest characters. 



Altogether the conclusion seems inevitable that all the British 

 specimens so far taken are members of the same species. The main 

 difficulties to my mind are the apparent similarity of Thomson's two 

 specimens (taken at different times in Weymouth Bay) and the close 

 similarity of two of ours. But the difficulties are not sufficient to 

 outweigh the evidence for the conclusion which I have just expressed; 

 variation in Nudibranchs frequently takes place in definite directions 

 (cf. Cavolina Farrani, No. 31, infra), and the Weymouth examples 

 and our two similar individuals may well represent two fixed varieties 

 of the species. It ought to be noted that these two individuals had ap- 

 parently been copulating, on account of their depositing eggs at the 

 same time. So far as the evidence goes, therefore, we have here a 

 species of Nudibranch producing offspring of very variable external 

 structure, the individuals apparently showing a tendency to unite 

 rather with those of their own variety than with those unlike them- 

 selves. 



I have to add that I regret to have been unable to consult Yerany's 

 account of his species Genei. In all probability it will be found that 

 he has not limited its characters to so extreme a degree as Alder 

 has done with regard to L, marmoratus, flavidus, and portlandicus, 

 and in this case our specimens would probably be referred to 

 L. Genei, Yerany. In the meantime I have thought it well to 

 adopt a new name, liom^anotus varians, for the single British species, 

 since to employ any one of Alder's names would lead to confusion. 



There is an observation which I omitted to add in its proper 

 place above, viz. that the first described individual was seen by 

 me occasionally to swim vigorously through the water in the dish 

 in which I had placed it, by lashing its body from side to side, much 

 as it is described by Lowe for a Nudibranch from Madeira of the 

 genus Peplidia (vide Alder and Hancock's Monograph, p. 22). The 

 two other individuals were torpid in their movements, and would 

 not expand sufficiently for me to be able to make out the exact 

 dimensions of the foot. 



