440 ANIMAL NATURE OF DIATOMEJE. 



the highest powers of the microscope. It seems as if 

 the so-called external spermatia of the Monnemce become 

 detached before development, as Kiitzing observed, in 

 one species; and that in the Sckizonema, on the con- 

 trary, these spermatia, if we regard their internal collo- 

 cation, become developed within the cavity of the 

 generating frond. 



This is one of the principal reasons why the deter- 

 mination of species becomes extremely difficult. Others 

 may be mentioned : the immense number of species ; the 

 excessive variations of external form ; the want of agree- 

 ment between the measurements indicated by Kiitzing 

 in his descriptions and those of his figures, and those 

 also deduced by direct observation from authentic speci- 

 mens ; finally, the very intricate synonymy, impossible to 

 disentangle when we have not before the eye, authentic 

 specimens of all the species, to institute a comparison. 



Kiitzing describes and figures twenty-four with won- 

 derful accuracy. To these I think that eighteen, ascribed 

 by him to the preceding genus, ought to be added ; and 

 not a few still remain to be denominated and described. 

 We will be as concise as possible. 



Schizonema implicatum, Harv. 

 Micromega intricatum, Kiitz. 



Kiitzing does not give any reason for changing the 

 name. 



Schizonema parisiticum, Griffiths. 



Kiitzing says the Navicula3 are ^th of a Paris line, or 

 0-0235 millim. He figures the largest of them 4 millim., 

 corresponding to 0' 009 5. In an authentic specimen 

 received from Berkeley, as well as in those from 

 Lenormand and Brebisson, under the name of 8. rutilans, 

 and corresponding exactly with the first, I find the 

 greatest length of the Naviculas 0'02 millim., and the 

 greatest breadth 0'005. 



