4 Prof. G. J. Allman on a new genus of Mollusca. 
longitudinalibus instructa. Tentacula duo tantum ante duos 
ocellos in vertice sita. Anus (et apertura sexualis ?) im medio 
dorso.” 
Ehrenberg’s genus includes but a single species, namely S. 
ornatus. 
It will here be seen that Stiliger is at once distinguished from 
the new genus by the presence of tentacula. In Lovén’s descrip- 
tion of S. modestus he gives as a character, “ tentacula lateralia, 
minuta tuberculiformia extrorsum vergentia.” The organs how- 
ever here called tentacula are certainly incorrectly so named ; 
they are merely lateral prolongations of the head. S. ornatus on 
the contrary has two long genuine tentacula situated on the ver- 
tex, “ tentacula duo longa ante oculos in vertice posita.” 
In Ehrenberg’s figure the anus is represented as occupying a 
position considerably anterior to that assumed by this orifice in 
Alderia.. In the latter it is placed near the posterior extremity 
of the body as in Doris, while in S. ornatus it is placed over that 
part of the animal usually occupied by the heart. This however, 
as Mr. Alder in a letter which I have from. him observes, would 
not perhaps of itself afford a character sufficient for generic sepa- 
ration. Altogether Ehrenberg’s figure is that of a very different- 
looking animal from Lovén’s Stiliger modestus, so that upon ma- 
ture deliberation, and with the full concurrence of Messrs. Alder 
and Hancock, I have determined upon the retention of the ge- 
nus which I originally established for the reception of the Nudi- 
branch of the Irish salt-marsh. 
It must always be borne in mind that the account now given. 
is the result of an examination of spirit specimens, and therefore 
necessarily imperfect ; the characters assumed as generic, how- 
ever, I believe to be indubitably established, and we must only 
look forward to the detection of recent specimens throwing ad- 
ditional light upon the structure of this most interesting little 
animal. 
The genus which it is necessary to construct for the reception 
of the new Nudibranch, I have great pleasure in dedicating to 
Joshua Alder, Esq., whose researches among this curious tribe of 
Mollusca constitute one of the many striking features by which 
modern zoological research is characterized ; and from whose la- 
bours, in conjunction with those of Mr, Hancock, natural science 
is now receiving so valuable a contribution in the beautiful work 
of these gentlemen on the British Nudibranchiate Mollusca. 
The following are the characters of the genus :— 
Noy. gen. ALDERIA. 
Gen. Char. Corpus oblongum. Capitis latera utrinque in lo- 
bum producta; ore maxillarum experti linguam armatam in- 
