ON MOUNTING. 145 



in marshy districts, on the surface of the ocean, or 

 attached to algae and shells, they abound in a living 

 and growing state, whilst their siliceous valves, after 

 the death of the cell-contents, are to be found, in a 

 more or less perfect condition, as deposits at the 

 bottom of rivers and ponds and on the sea floor. 

 From the stomachs of holothurians, ascidians, mol- 

 luscs, and of various shell-fish, exquisite varieties 

 have been obtained in profusion. As fossils they 

 form beds of enormous extent, intermixed with the 

 siliceous casts of polycystina, radiolarians, &c., and 

 the calcareous shells of foraminifera, whilst fossil 

 deposits purely diatomaceous and yielding the most 

 beautiful, rare and varied forms have been dis- 

 covered in all parts of our globe. In order to clean 

 diatoms and render them fit for mounting, as either 

 " selected " or " strewed " slides, considerable ex- 

 perience and great care are necessary. The sub- 

 siliceous varieties especially require very delicate 

 treatment, whilst with many of the fossil and recent 

 forms, which are intensely siliceous, altogether 

 " heroic " and drastic measures must be resorted to 

 in order to ensure perfect cleansing of the surfaces 

 of the valves. There have been nearly as many 

 nostra propounded and published for the cleaning 

 and mounting of diatoms as there are genera and 

 species of the diatomaceas themselves. Years of 

 experience and constant experiments have led the 

 author to the conclusion that the simplest and 

 safest modes of treatment are the best. For living 

 diatoms and especially for the sub-siliceous forms 

 (e.g., pleurosigma) the following method will give 

 perfect results : Remove all dirt, debris, and salt, 

 by repeated washings, thoroughly shaking up the 

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