362 ANIMAL NATUEE OF DlATOMEyE. 



The mode of union is unknown. But the existence of 

 a kind of articulation which permits an opening and 

 closing, like the valves of a shell-fish, described by Corda 

 in a species of Surirella, has been denied by other 

 observers. Be this as it may, whether spontaneous after 

 death, or induced by external means, this separation 

 does take place in a regular manner. Now, if we sup- 

 pose an organic cell with a wall permeated by silica, and 

 with a four-sided figure, we can easily suppose that all 

 the sides will mechanically support each other. More- 

 over, we shall meet with numerous facts by a different 

 kind of analogy, viz., that with soHd animal tissues belong- 

 ing either to the internal skeleton or the external tegu- 

 ment. 



The four valves are equal in length, but in many 

 species and genera one pair exceeds the opposite pair in 

 breadth. In order to establish an uniform language it is 

 convenient to term those primary valves or surfaces 

 which exhibit along the middle the line of division in 

 the act of deduplication, which, since it is formed here in a 

 normal manner, runs parallel to the other two surfaces, 

 denominated lateral. Along the primary surfaces we 

 frequently see longitudinal lines, which terminate at the 

 two extremities in small apertures. From their internal 

 surface there project into the cavity linear marks vari- 

 ously formed but always longitudinal ; these are termed 

 vittce. 



The lateral surfaces have frequently a round aperture, 

 of greater or smaller size, in the centre, and from this a 

 fissure extends towards each extremity. This fissure 

 either loses itself gradually, or expands into the regular 

 terminal apertures. When this occurs, each of these 

 surfaces is divided into two distinct valves. On these 

 lateral surfaces we observe the striae, lines, and trans- 

 verse costge, no less admirable for their beautiful appear- 

 ance than for their constant regularity in number, 

 direction, and proportion. When many individuals are 

 united together to form one compound being, like a polyp, 



