ANIMAL NATURE OF DIATOMEiE. 385 



10. Cyclotella. — Tndividua singularia vel binatim 

 conjuncta, disciformia ; latus primarium distinctum, an- 

 nuliforme ; lafera secondaria plana. {Lorica bivalvis, 

 valvis planis, orbicularibm, annulo interstiali conjunitis.) 



With this genus there commences a series of forms 

 which have a type totally different from the preceding. 

 In the former the two primary surfaces are always dis- 

 tinct, whilst in those under consideration they unite in a 

 continuous cylindrical superficies. In the one the lateral 

 surfaces have always one diameter larger than the other, 

 so that they become more or less lengthened ; whilst in 

 the other they are flat and circular, or convex and con- 

 cave, but always segments of a sphere. In the preceding 

 series we have, indeed, among the Odontidia, the Fragil- 

 laricE, and Diatovice, such a convexity of the principal 

 surfaces as is almost round, but the extremities remain 

 always distinct; and in the lateral surfaces the striae 

 or any other processes run transversely without in- 

 terruption. Here, on the other hand, we have a very 

 evident radiated arrangement. This same arrangement 

 is presented in another family (Coscinodiscese) of another 

 group ; and there we shall have occasion to mention it 

 again. In the genus Cyclotella we find species brought 

 together which in fact possess great similitude of external 

 form. But those from fresh water (C. operculata, C. 

 Meneghiniana,) are free from attachment, and surrounded 

 by a gelatinous substance; whilst the marine species 

 (C. scotica, C. ligustica, C. maxima^ are parasitic. But 

 if, again, we observe that the striae or radiating puncta of 

 the former are wanting in the latter, we shall find, in 

 that circumstance, I'eason to suspect that even in internal 

 organisation, to us entirely unknown, there may be very 

 remarkable differences. Hence the observations of Nageli, 

 on what he terms a new species of Gallionella, and 

 which certainly ought to be referred to this genus, are 

 very valuable. (See Henfrey's translation of Nageli's 

 ' Memoir on Vegetable Cells,' in Ray Society's Reports, 

 1846, Tab. VI, figs. 1, 2, 3; Tab. VII, figs. 27, 28.) 



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