422 THE MICROSCOPE. 



sisting of thin walled vesicles filled with air. If some of 

 the deposit be crushed between two pieces of glass, and 

 examined with a power of about 300 diameters, a marking 

 will be perceived on the outer or convex surface of many of 

 these vesicles, similar to that of many Diatomacea?, such 

 as Pleurosigma, Coscinodiscus, &c. Hounded elevations 

 more or less hexagonal at the base, and more or less 

 regularly arranged, cover the surface of the siliceous 

 pellicle, and not unfrequently this kind of marking is so 

 regular as to give the fragments exactly the appearance of 

 portions of diatomaceous valves. 



This remarkable circumstance attracted the attention of 

 Prolessor Max Sehultze, who devoted a great deal of time 

 to the investigation of the subject, and has recorded in a 

 voluminous paper 1 the results of his observations. He 

 says, " The appearances presented under the microscope by 

 the siliceous pellicles were such as to suggest that they 

 were due possibly to crystallisation. The minute eleva- 

 tions on the surface, when viewed on the side, often appear 

 sharply acuminate, so as readily to convey the impression 

 that they are formed by minute crystals of silex ; and this 

 impression is strengthened at first sight by their sharply 

 defined hexagonal basis, when viewed vertically. The 

 circumstance, again, that these elevations are sometimes 

 rounded at the summit and circular at the base, might be 

 attributed to the accidental interference of free hydi-orluo- 

 silicic acid, &c. But experiments to eleminate the action 

 of this agent showed that it had nothing to do with the 

 variety of appearance in the elevations. 



" Most of the species of the diatoniacese are characterised 

 by the presence on their outer surface of certain differences 

 of relief, referable either to elevations or to depressions 

 disposed in rows. The opinions of rnicroscopists with re- 

 spect to the nature of this marking are divided. Whilst 

 in the larger forms, and those distinguished by their coarser 

 dots, the appearance is manifestly due to the existence of 

 thinner spots in the valve, we can not so easily explain the 

 cause of the striation or punctation in Pleurosigma 

 angulatum and similar finely-marked forms. In these it 



(1) " Verhandl. d. Natur Hist. Vereinsder Preussisch. Rheinland, u. West- 

 phd." JaJir xx. p. 1. Micros. Jour. Science, vol. iii. p. 120. 



