238 Transactions of the [Sess. 



IX.— DIATOMS. 



By Mr JOHN RATTRAY, M.A., B.Sc. 



Scottish Marine Station, Granton. 



{Read Feb. 26, 1885.) 



Among the vast and varied array of micro-organisms with which 

 the student of nature has to deal, there are perhaps none that offer 

 more attractive and interesting features than those presented by 

 the DiatomacCce. These unicellular organisms, with their charac- 

 teristic and, in most cases, delicately-sculptured siliceous walls, 

 are to be found in fresh and salt or brackish water, as well as on 

 damp ground generally, either in a free state or not unfrequently 

 adhering to stones or to decaying or healthy plants of larger size 

 than themselves. Viewed under a moderately high power, they 

 are found to possess a very great variety of outline — from the 

 narrow elongated Nitzschice and Synedrse to the cuneiform or wedge- 

 shaped Meridions, the discoidal Coscinodisci, the curved and saddle- 

 shaped Campylodisci, the sigmoid Pleurosigmse, the navicular 

 Cocconees, the tri- or poly-angular Triceratia, and the geniculate 

 Achnanthidia. Appendages of various forms, and presenting 

 various differences as to disposition and dimensions, are also not 

 uncommon. These occur sometimes "in the form of marginal or 

 submarginal alee, as in Surirella ; sometimes as elevated keel-like 

 ridges, as in Nitzschia ; and sometimes as tubular horns, which 

 may be simple or branched, as in Chtetoceros or Rhizosolenia. But 

 throughout the wide range of external configuration, a remarkable 

 symmetry is, with few exceptions, — such as Achnanthes and some 

 others, — generally very apparent. This significant characteristic 

 at once forces itself upon the attention, the entire Diatom cell 

 being composed essentially of two'- valves and a connecting hoop 

 or cingulum intervening between the former. The valves are 

 in most cases similar to one another, but the cingulum may vary 

 very greatly in its appearance — that is, in its state of development ; 

 and it is in all cases of the utmost importance to remember that 

 the outline of any given frustule need not present the same con- 

 tour when viewed with the valves next the eye — i.e.., from the 

 valval or side aspect — and with the cingulum next the observer 

 — 2.e., from the zonal or front aspect. No more striking contrast 

 in this respect need be sought than that which is found in the case 



' In some cases pseudo-multiple frustules are found : thus, a Triceratium 

 breaks up into two triangular end plates and tliree rectangular side plates, 

 while the latter may again divide into several paralleliform pieces. 



