132 SMITH, ON THE DETERMINATION OF 



but unascertained period, reach the mature form of the 

 ordinary frustule. 



The farther growth of the individual cell seems now to be 

 almost entirely arrested by the formation of the silicious 

 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 idiosyncracy of the 

 embryo, or upon the circumstances in which its embryonic 

 growth takes place ; consequently a very conspicuous diversity 

 in their relative magnitudes may be usually noticed in any 

 large aggregation of individuals, or in the same species col- 

 lected in different localities. 



It may also be easily conceived, that while a typical outline 

 of its cell must be the characteristic of a certain species, such 

 outline may to some extent be modified by the accidental 

 circumstances which surround the embryo during its earlier 

 growth and development. A lanceolate form may become 

 linear, elliptical, or even somewhat oval, by the pressure of 

 surrounding cells ; and acute ends may be transformed into 

 obtuse or rounded extremities. 



Those who understand the process of self-division will see 

 here a sufficient reason for the occurrence of multitudes of 

 frustules, deviating from the normal form, or even for the 

 existence of myriads, at one spot, all having a form different 

 from the type, the single embryo from which they have all 

 sprung by self-division (which process stereotypes the shape 

 with which it commences), having from some accidental cir- 

 cumstances become modified in its outline. 



It follows then, from these considerations, that neither size 

 nor outline are sufficient to enable the observer to determine 

 the species of a Diatomaceous frustule. If he has the means 

 of comparing specimens in sufficient numbers, and from 

 various localities, he may fix with tolerable certainty upon 

 the magnitude and form which may be regarded as the 

 average and type of the species ; but without such oppor- 

 tunities, a reliance upon such characters will inevitably lead 

 to the undue multiplication of species, and to a confused and 

 erroneous nomenclature. 



There are three circumstances which seem to me to be of 

 essential importance as regards the specific character of the 

 diatom : first, the structure of the valve ; second, the habitat ; 

 and third, the arrangement of the endochrome in the living 

 frustule. I have placed the structure of the valve first in 

 order, because this is a feature which can be ascertained in 



