DIATOM A CE.K 



603 



highly probable that many of the forms at present considered as dis- 

 tinct from each other would prove to be but different states of the 

 same if their whole history were ascertained. On the other hand, 

 it is by no means impossible that some which appear to be nearly 

 related in the structure of their frustules and in their^mode of 

 growth may prove to have quite different modes of reproduction. 

 At present, therefore, any classification must be merely provisional ; 

 and in the notice now to be taken of some of the most interesting 

 forms of the Diatomacece, the method of Professor Kiitzing, which 

 is based upon the characters of the individual frustules, is followed, 

 in preference to that of Mr. W. ^Smith, which was founded on 

 the degree of connection remaining between the several frustules 

 after binary division. 1 In each family the frustules may exist under 

 four conditions : (a) free, the binary division being entire, so that the 

 frustules separate as soon as the process has been completed ; (b) 

 stipitate, the frustules being implanted upon a common stem (fig. 



FIG. 44. Meridian circulare. 



FIG. 449. Bacillaria paradoxa. 



450), which keeps them in mutual connection after they have them- 

 selves undergone a complete binary division ; (c) united in a filament, 

 which will be continuous (fig. 445, A, B) if the cohesion extend to 

 the entire surfaces of the sides of the frustules, but may be a mere 

 zigzag chain (fig. 451) if the cohesion be limited to their angles; 

 (d) aggregated into a frond (fig. 464), which consists of numerous 

 frustules more or less regularly enclosed in a gelatinous investment. 

 Commencing with the last-named division (A), the first family 



tion produce ^V. splendida, which gives rise to N. bifrons by the same process. He is 

 only able to speak positively, however, as to the production of N. bifrons from N. 

 splendida; that of Surirella microcora from N. bifrons, and that of N. splendida 

 from Surirella microcora, being matters of inference from the phenomena witnessed 

 by him. 



1 The method of Kiitzing was the one followed, with some modification, by Mr. 

 Ralfs in his revision of the group for the fourth edition of Pritchard's Infusoria ; 

 and to his systematic arrangement the Author would refer such as desire more 

 detailed information. 



