XXVIU INTRODUCTION. 



different aspects of the frustule. It would have been desirable 

 to have here adopted the terms employed by former writers, 

 especially by those distinguished authors who have led the 

 way in the study of these organisms ; but this course is pre- 

 cluded by the circumstance that these writers have employed 

 terms which imply views of the structm-e and nature of the 

 Diatomaceous frustule that are now altogether inadmissible, or 

 at all events at variance with the conclusions of the present 

 writer. 



Thus, the terms ventral and dorsal, employed by Ehrenberg, 

 would be clearly inconsistent, if not unmeaning, when applied 

 to the different aspects, or parts of a plant. The terms primary 

 and secondary sides, adopted by Kiitzing and others, are not 

 open to the same difficulty ; but they labom- under this objection, 

 that they have not been employed by Mr. Ralfs, the highest 

 English authority upon the subject. I shall therefore adopt the 

 nomenclature of the last-named writer, as the most convenient 

 for the English student, and use the term " front view," to de- 

 note the aspect of the frustule. when the valvular suture, or the 

 line along which seK-division takes place, is turned towards the 

 observer ; and the term " side view," when the centre of one 

 valve is directed to the eye. 



Even these terms will require modification when applied to 

 some of the more complex and irregular forms ; but in general 

 their meaning will be sufficiently obvious, and special cases will 

 be noticed as they present themselves. 



Self-division also supplies circumstances and distinctions, 

 which appear to me most suitable in the present state of our 

 knowledge, on which to found a generic arrangement of the 

 Diatomacese. The circumstances which accompany the Repro- 

 duction of these organisms are so imperfectly ascertained, and 

 that in so few species, that it is impossible to employ them with 

 advantage in a generic arrangement. Self-division seems to me 

 to come next in order, as a most important function connected 

 with increase and growth, and to supply the necessary variety 

 of phoenomena on which to ground our sectional divisions. 



T have therefore separated those forms where self-division is 

 accompanied by the secretion of a permanent gelatinous or 



