INTRODUCTION. XXV 



of the conjugating process, will produce frustules with the same 

 specific characters as themselves. In describing a species, we 

 should therefore carefully note the character of the striation, and 

 state as nearly as possible the average distance of the striae. 

 The dry valve will frequently aid in such determination, the 

 presence and extent of the striation being usually indicated by 

 the colour of the surface, and the differences in colour not un- 

 frequently answering to the relative distances or distinctness of 

 the strise. 



Another feature, which appears in its modifications to confer 

 a specific character upon the Diatomaceous organism, is the 

 arrangement of the endochrome or cell-contents in the living 

 frustule. These modifications are familiar to those who are 

 accustomed to the examination of freshly gathered specimens of 

 the Diatomaceae, and are found to be constant and characteristic 

 of each specific form. In one case the endochrome is closely 

 applied to the inner surface of the valve ; in another, aggregated 

 in the centre of the frustule ;. sometimes sparingly diffused 

 throughout the interior, or again exhibiting a radiate or stellate 

 arrangement; at all times having one or several oily (?) glob'ules 

 which occupy in different species different positions, but are 

 constant in number and situation in the same species. 



Should both these methods of determining the species fail the 

 inquirer, there remains a third which will often aid him in the 

 desired discrimination, viz. the locality in which the specimen 

 has been discovered. I feel persuaded that a marine species 

 will not nourish under fluviatile influences, nor a freshwater 

 form long survive when transferred to a marine habitat. Still 

 further, I believe that certain species are far more special in 

 their tastes, some selecting mountain-torrents, others clear and 

 still waters ; some preferring the deltas of rivers, and others 

 fixing their habitation in boggy pools or alpine lakes; some 

 being exclusively littoral, and others found only in the deeper 

 parts of the sea. 



Where structural differences are not obvious, when striation 

 is indistinct or too minute for detection, and the arrangement 

 of the cell-contents is modified by the death of the frustule, a 

 knowledge of the habitat of the specimen, when in a living state, 



