ANIMAL NATURE OF DIATOME.E. 469 



they are more closely allied to the Fragillariese than to 

 any other preceding family. 



Kiitzing rightly observes, that the StriatelleaB are 

 found indifferently, sometimes free, sometimes affixed. It 

 seems, however, that they were all originally in the 

 second of these conditions. Still, in the association of 

 individuals resulting from deduplication the Striatelleas 

 repeat all the forms of the Fragillarieae. Finally, as to the 

 interanea, Kiitzing says they are at first disposed uni- 

 formly, then collected into small spherules, which become 

 condensed into a central globe. This last condition is 

 characteristic of the single species of the genus Striatella. 

 But with regard to Hyalosira and Rhabdomena, I think it 

 important to mention the wasting away of the coloured 

 internal substance into longitudinal strips parallel and 

 intermediate to the vittse. 



54. TETRACYCLUS. 'Bacilli late tabulatia injilum arete 

 connati, lonyitudinaliter continue et arete vittati; a latere 

 secundario utroque apice late rotimdati, ventro media 

 maxime inflato, stipes nullus. 



The presence of vittae solely distinguishes the single 

 species (T. lacustris) from Odontidium. The lateral sur- 

 faces are convex, and traversed by transverse and arcuate 

 costae. I have not seen the straight median costa figured 

 by Hassall, nor the absence of these costae in the space in 

 the centre, as figured by Kiitzing. But because the 

 median lobe is strongly convex, its plane is different from 

 that of the lateral lobes ; hence, when the costa? of one 

 can be seen, those of the other cannot, and vice versa. I 

 can in no way comprehend Kiitzing's reason for enume- 

 rating tkis genus among the Diatomea stomatica, except 

 by supposing that he regarded as a large central aperture 

 the entire median cavity, which seems to be open when, 

 observing the lateral surface too nearly, the corresponding 

 convexity escapes from the field of view. Fragments of 

 organisms, similar to Tetracyclus, are very common in the 

 fossil flour of Santa Fiore. 



