134 SMITH, ON THE DETERMINATION OF 



lakes ; some being littoral, and others only found in the 

 deeper parts of the ocean. When structural differences are 

 not obvious, such circumstances as these will not unfrequently 

 assist the inquirer to resolve his doubts, and assign to a form, 

 a position and a name. 



And lastly, the arrangement of the endochrome, in the living 

 frustule, will be found to confer a specific character, one far 

 more certain than habitat ; but from its being less easily, or 

 less frequently within the observer's notice, not so practical 

 in its application. 



Those who have the means of examining the living cell are 

 well aware that there is in every species a distinct feature 

 conferred by the position of the cell contents ; in one the en- 

 dochrome 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 exhibit- 

 ing a radiate or stellate arrangement ; at all times having one 

 or several oily globules, which occupy in different species 

 different positions, but are constant in number and situation 

 in the same species. 



These characters are not always within reach of the ordi- 

 nary microscopist, who contents himself with an examination 

 of the silicious valve in a dried or prepared condition ; but 

 the systematist, who desires to define and discriminate, will 

 not neglect these features, and will hesitate to decide between 

 closely allied forms until he has the means of scrutinizing the 

 living frustules, and noting the differences which exist in the 

 position and arrangement of their cell contents. 



It will be apparent from these remarks, that the determina- 

 tion of species in the Diatomace2e is at all times a task of 

 difficulty, and that this difficulty is much enhanced when the 

 observer does not enjoy the opportunity of examining these 

 organisms in their living state, or has only prepared or fossil 

 specimens, within the reach of his observations. 



In such a case the student is disposed to rely too much 

 upon the obvious characteristics of size and form ; now it 

 often requires a careful examination of specimens collected in 

 various localities, and in every condition of growth, to enable 

 the observer to fix upon the size which may be regarded as 

 an average, or upon the outline which ought to be accepted 

 as typical of the species. 



To adopt as specific a shape or size that occurs in but 

 one gathering, whether the individuals therein be more or 

 less numerous, is, I feel persuaded, a most fallacious method 

 of procedure. And although the striation is at all times an 

 important guide, it often happens that this feature is so nearly 



