SPECIES IN THE DIATOMACEiE. 133 



«very condition under which these forms present themselves, 

 whether in a living, dried, or fossil state ; and also because it 

 appears to me to be the most constant and obvious character 

 of such organisms. These varieties of structure arise from 

 the modes in which the silex combines with the cellulose of 

 the epiderm, and this combination seems to follow certain 

 and invariable laws which are subject to no derangement from 

 the external circumstances in which the growth of the embryo 

 may take place. The structure of the valve reveals itself in 

 the striation, in the character of the striation, may there 

 fore be found a good specific distinction. Whether the striae 

 are costate or moniliform, parallel or radiate, reach the 

 median line, or are absent from a greater or lesser portion of 

 the valvular surface, and numerous other features which may 

 exist, separately or in combination, — these appear to me to be 

 circumstances which the observer who wishes to discriminate 

 and determine the species should most carefully regard. The 

 relative distances of the striae, and their greater or less dis- 

 tinctness, (which are not, by-the-bye, always identical cha- 

 racteristics,) are also features which may be safely and 

 properly recorded ; but I am not certain that these features 

 may not, to a slight extent, be modified by localities and age, 

 and am disposed to believe that they are certain guides only 

 when we have made allowance for these conditions ; and that, 

 while they are constant in frustules originally from the same 

 embryo, they may slightly vary in those which owe their birth 

 to different embryonic cells. 



In describing a species, we should therefore carefully note 

 the character of the striation, and state as nearly as possible 

 the average number of the striae. The dry valve will frequently 

 aid in such determinations, the presence and extent of the 

 striation being usually indicated by the colour of the valvular 

 surface, even when the power employed fails to detect the 

 striae themselves ; the differences in colour not unfrequently 

 answering to the relative distances, or distinctness of the 

 markings. 



Next to the striation of the valve I have placed locality, 

 as a means of arriving at a specific distinction. Where forms 

 are closely allied, this circumstance will often aid in their dis- 

 crimination ; for not only do I regard it as established that no 

 fresh-water species will live in a marine habitat, nor a marine 

 species flourish under the predominating influences of springs 

 and rivers ; but I believe that certain species are far more 

 special in their tastes, some selecting mountain torrents, others 

 clear and still waters ; some rejoicing in the deltas of rivers, 

 and others fixing their habitations in boggy pools, or alpine 



