414 ANIMAL NATURE OF DIATOME^B. 



(Denticula, Synedra,} or in some species (Surirella) of 

 these we must resort to other characters to establish the 

 distinction, more especially as the forms are frequently 

 very similar. This essential character that of the entire 

 family is the presence of a central aperture in both the 

 secondary surfaces. But this character is wanting in some 

 species of Namcula (Oxyphyllum, vulpina, &c.) and in one 

 of the subsequent genera ; and Kiitzing observes that it 

 is often very difficult to discover the aperture on account 

 of its minuteness, and that it is absent in some, though 

 evident in others of the same species. Therefore, without 

 attempting from this to argue in opposition to the 

 organographical importance of the character itself, it is 

 certainly of diminished value systematically considered ; 

 and Kiitzing acted prudently, in doubtful cases, by 

 regarding evident affinities of figure, so as not to separate 

 similar objects from one another. Kiitzing divides the 

 large number of species (137) into six groups, according 

 to their shape; lanceolate, oUongce /. elliptic^, gibbce, 

 constricts s. nodosce, lunate, sigmoidece. The greater 

 number of species belonging to the first section {lanceo- 

 late} have precisely the form termed navicular, the 

 primary surfaces linear, and the secondary longato- 

 elliptical, with their apices more or less acute. We have 

 seen above that the first section of Synedrce (Scaphularite) 

 have this same form ; and after the admission of Kiitzing 

 himself and his example in respect to one species, it is 

 truly surprising to see the Scaphularia and Navicute 

 lanceolate generically separated from each other. Some 

 of the species referred to this first section show a gradual 

 transition to the different forms of the succeeding sec- 

 tions. But in all these we find species described and 

 figured in Kiitzing's work so similar to one another, that 

 there occurs a well-grounded suspicion respecting the 

 propriety of the distinction. And here it is proper to 

 observe, that in Diatomeae, more perhaps than in any 

 other class of organised beings, it is difficult to pronounce 

 a certain decision on the value of characters. In animals, 



