106 MEMORANDA. 



the one for the other. The opticians often give to N. affinis 

 the name of N. aniici, no doubt because this diatom was the 

 favourite test of that able micrographer. N. affinis is also 

 confounded with the A^. gracilis, N. rhombica, N. cuspidata, 

 &c., in such a way that it is sometimes difficult to recognise 

 them. I have said that the two diatoms in question ought 

 not to be confounded. In fact, whilst the N. affinis, with the 

 elliptic valve, is pinched up towards its ends, it is quite other- 

 wise with N. rhomboides, which has a nearly quadrangular 

 form, and the ends of which are lanceolate. The striae of this 

 diatom (85 in "OOl") make it a test of the first order. What 

 astonishes me is that certain authors of consideration, such 

 as MM. Arthur Chevalier, Henri Van Heurck, Heinrick 

 Frey, and many others, have not given to the diatom, which 

 they describe as the N. affinis, or test of Amici, its real name. 

 Lastly, it appears that M. de Brebisson, the able French 

 micrographer, in a new work, which he is preparing on the 

 diatoms, has dedicated to one of these authors, M. Henri 

 Van Heurck, a genus Vanheurckia, which ought to com- 

 prise N. rhomboides, crassinervis, cuspidata, ambigua, collet, 

 viridum, and vidgare Perhaps this will preserve us from the 

 approach of complete confusion, — Mouchet, Rochefort-sur- 

 mer. 



Corethra plumicornis. — The note on the Bibliography of 

 this interesting insect and its larvse, which appeai'ed in the 

 Notes and Correspondence of the October number of the 

 ' Journal,' in which number, also, Professor Jones's paper 

 appeared, should have been signed '' T. Rymer Jones," since 

 it was sent for publication to the Editors by that gentleman. 



Note on a Proposed Form of Condenser. — By the intersec- 

 tion at right angles of two equal and similar half-cylinders, 

 whose flat sides are in the same plane, a solid is formed, 

 which is represeiited in the accompanying figure. 



