FRA GIL ARIA. 75 



the form of a spindle. B. cuneata (Drop I. 

 fig. 18) is of wonderful construction each in- 

 dividual is wedge-shaped, and they still preserve 

 the straight ribbon or chain-like form, by being- 

 placed alternately the broad and narrow end 

 together. These chains of the Bacittaria are free 

 and floating ; as well as those of the next genus, 

 Fragilaria, specimens of which are frequently 

 seen to rise in the water, and turn round ; the 

 individuals also, when separated, move forward 

 gently. They may be distinguished from Na- 

 vicida by being square at the ends, instead of 

 pointed, and having two openings instead of one. 

 In F. yrandis there are as many as thirty indi- 

 viduals found linked together. F. rhabdosoma 

 (Drop I. fig. 19) is very slender, and the bands 



