244 Van Beneden on the genera Eleutheria and Synhydra. 



in a drop of water and covered with a top glass. Now it is a very 

 great advantage to be able at once to mount anything without 

 being obliged to shift the top glass in order to introduce a different 

 fluid ; in doing this the relative positions of parts are frequently 

 changed^ or in the case of animalcules the latter may be lost^ and 

 many a rare thing have I irrecoverably been deprived of by ha- 

 ving recourse to a different fluid before I was led to adopt my 

 present mode. If any person could discover a method of mount- 

 ing marine Algse so as to prevent the loss of the beautiful tints 

 in many of the more delicate, he would confer a great boon on 

 microscopists. I regret to say that every method of preserving 

 colour in the Griffithsice, the Calithamnia and Ceramia have with 

 me signally failed ; their delicate rose-coloured tints are soon lost, 

 even though the cell is allowed to dry and the preparation ever 

 afterwards kept in a dark place. 



Inclosed I forward to you a cell, also a slider containing a 

 portion of Batrachospermum vagum according to Sowerby, but 

 in my humble opinion a variety only of B. moniliforme, together 

 with the spiral vessels of the garden Nasturtium. These are 

 all prepared in the manner detailed above, have been mounted 

 nearly a year, and have suffered no change during this time. 

 I have the honour to be, Sir, your obedient servant, 



William Reckitt, M.R.C.S.L. 



Boston, Lincolnshire. 



XXX. — On the genera Eleutheria and Synhydra. By P. J. Van 

 Beneden, Professor at the University of Louvain*. 



In order that physiological researches may extend the state of 

 our knowledge in zoology, it is requisite that the limits of the 

 genera and species composing the scale of beings should be well 

 determined. The object of the naturalist should be to become 

 acquainted with the animal in the different phases of its develop- 

 ment. A celebrated professor has said, that we do not know a 

 species, if we have not studied it from its exit from the q^^ up 

 to the period of its decrepitude. 



M. de Quatrefages has communicated to the French Academy 

 of Sciences, a memoir on a new animal which he has called Eleu- 

 theria. While our work on the Tubularics was in the press, we 

 received this memoir f, and we could not but express a doubt 

 of the zoological value of this new genus. It might indeed be a 

 young animal, we said, which in the adult state would come to 



* From the Bulletin de I'Acad. Roy. de Bruxelles, vol. xi. no. 10. Trans- 

 lated from a separate impression kindly furnished by the author. — Ed. 

 f Annales des Sciences Naturelles, 2nd series, tom. xviii. p. 270. 



