Aye lla 
See 
Prof. G. J. Allman on a new genus of Mollusca. 3 
blished genera, and I accordingly noticed it at the York Meeting 
of the British Association in Sept. 1844, under the name of A/- 
deria amphibia, an appellation under which it has been also re- 
corded by Mr. Thompson in his Report on the Invertebrate 
Fauna of Ireland, and by Messrs. Alder and Hancock in their 
Report on the British Nudibranchiate Mollusca. 
Shortly after my noticing the little animal in question as a 
new genus of Mollusca, I received a letter from Mr. Alder, ac- 
companying an extract from a paper by Dr. Lovén of Stockholm, 
which had just appeared in a foreign periodical, and which con- 
tains an account of a Nudibranchiate mollusk referred by Lovén 
to the genus Sétliger, Ehrenberg, and described by the Swedish 
naturalist under the name of S. modestus. 
On comparing Lovén’s description of S. modestus with the sub- 
ject of the present communication, it was evident that Lovén’s 
animal and the Nudibranch of the Irish salt-marsh were the 
same. A reference however to the characters of Ehrenberg’s 
genus Séiliger, rendered it by no means so apparent that Lovén 
was correct in the generic location of his mollusk. 
Under this difficulty Mr. Alder received a letter from Dr. Lovén, 
in which is the following passage respecting S. modestus :— 
“A very rare animal. It is now ten years since I found m 
two specimens, one of which was lost by accident. Fortunately 
I described and figured it alive, for I never met with any more. 
Stiliger, Ehren., has only one species, and as it requires but little 
to widen its characters enough to let in my new species, I thought 
it advisable to do so, and still think I was right in so'doing. At 
least I have not a superfluous genus on my conscience. Nothing 
is more easy than to make new genera, but the question is to 
find out the true generic characters, which, particularly in the 
Nudibranchia, is rather difficult.” 
On the above passage, Mr. Alder in a letter to me remarks: 
“So far Dr. Lovén; and after his opinion, you will perhaps 
scarcely venture to institute your new genus, though Mr. Han- 
cock and I, after mature consideration, think you would be right 
in domg so.” With the English naturalists 1 agree, and my ori- 
ginal opinion with respect to the necessity of a new genus for the 
reception of the salt-marsh Nudibranch remains unaltered. The 
characters indeed of this mollusk can scarcely be confounded with 
those of Stiliger, and in order that this matter may be made more 
apparent, I here subjoin Ehrenberg’s characters of the last-men- 
tioned genus from the ‘Symbol Physice.’ 
“ STILIGER, nov. gen. 
“Gen. Char. Habitus Eolidie. Corpus oblongum, pallio dis- 
ereto nullo. Latera corporis branchiarum stiliformium seriebus 
B2 
