INTRODUCTION. XX111 



ceous valves ; and the multiplication of these forms, of the size 

 thus reached, goes on with inconceivable rapidity by means of 

 self-division. 



The size of the mature frustule before self-division com- 

 mences, is, however, dependent upon the idiosyncrasy of the 

 embryo, or upon the circumstances in which its growth takes 

 place ; consequently a very great diversity in their relative mag- 

 nitudes may be noticed in any large aggregation of individuals, 

 or in the same species gathered in different localities. More- 

 over, while a typical outline of its frustule is the general cha- 

 racteristic of a species, this outline may be modified by the 

 accidental circumstances which surround the embryo during its 

 growth, and the development of its siliceous epiderm. 



The process of self-division now intervening, and necessarily 

 stereotyping the shape with which it commences, multitudes of 

 frustules slightly deviating from the normal form are sub- 

 sequently produced, so that the observer, judging from a single 

 gathering, may be led to fix upon a variety as representing the 

 typical form and size of the species. 



It follows from these circumstances that neither size nor out- 

 line is sufficient to enable the observer under ordinary circum- 

 stances to determine the species of a Diatomaceous frustule : 

 well-marked diversities of shape, depending upon structural 

 peculiarities, may be an auxiliary guide in the adoption of 

 generic distinctions ; but the slighter and accidental variations, 

 which occur during the development of the individual frustules, 

 forbid us to employ such characters in the determination of 

 specific forms. If the observer have the means of comparing 

 specimens in sufficient numbers and from various localities, he 

 may fix with tolerable accuracy upon the magnitude and form 

 which may be regarded as the average and type of the species ; 

 but without these opportunities, a reliance upon such characters 

 will lead to an undue multiplication of species, and to a confu- 

 sion and indefiniteness in their arrangement that will embarrass 

 and mislead the future inquirer. 



One specialty in the Diatomaceous frustule, presenting im- 

 portant modifications that may be introduced with propriety 

 into our generic descriptions, and at the same time offering -^-^Tr? 



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