Bacillaria.] THE INFUSORIA. 175 



other characteristics, that it is more proper to separate 

 them from this family. In Navicula alone has an undi- 

 vided locomotive organ been seen. This organ possesses 

 the power of moving in any direction, and accommodating 

 itself to any form similar to that curious muscular organ, 

 the tongue, in animals, or the foot-like process of snails. 



The shells of these creatures are often sculptured with 

 deep flutings ; where this occurs, the inside of the shell is 

 not always smooth, but follows the form of the exterior : 

 thus the strength is greatly increased, while the quantity 

 of solid material employed in their construction is not 

 augmented. The flutings in the living specimens being 

 filled with coloured matter, are discerned with difficulty, 

 and hence they are almost unknown. In the fossil state, 

 in which vast numbers of these are found, the shells are 

 empty, and in this state the flutings and other indenta- 

 tions become distinctly visible. I have, therefore, in the 

 engravings illustrating this family, selected drawings of 

 several in both states, so that the reader may form a clear 

 conception of their true characters, while the interest' 

 attached to the family generally is so great, and we possess 

 so few drawings of its various species, that I have been 

 induced to extend the number of illustrations consider- 

 ably. The order of their arrangement differs from that 

 given in the plates of Die Infusionsthierchen ; in that work 

 the species and genera are placed in plates indiscrimi- 

 nately, and no regular arrangement is observed. As some 

 fossil specimens have been discovered since Ehrenberg's 

 great work appeared, I have had drawings of the finest 

 I could procure made for me ; some of them are the 

 last productions of my friend, the late F. Bauer, Esq. ; 



