92 TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 



and Count Castracane repeated it a few years back. More recent 

 diatomists have not confirmed these assertions, and Mr. J. Schumann 

 was the first to establish (') that the striation is variable, and that the 

 number of striae of any species slowly increases, according to the elevation 

 of the locality in which the particular specimen is found. 



Schumann deduced from his oft repeated researches that temperature 

 was the most essential cause of variation in the striation. He established 

 that for the same species the number of striae so nearly corresponds with 

 the temperature that, for a given elevation, the temperature of the locality 

 can be so accurately determined by the number of stripe as to make 

 a thermometer quite superfluous. 



Pursuing his researches still further, Schumann was able to dispense 

 with the barometer, and to determine the heights of mountains by 

 examining the striae of certain species, which he had previously studied 

 in various localities of different heights above the sea level. 



The assertions of Schumann have been lately confirmed by Frere 

 Heribaud, of Clermont Ferrand ( 2 ) who has also studied the effects of 

 light on the striation of valves which have been gathered from lake- 

 bottoms. The author concludes his memoir thus : 



istly. Under the influence of a feeble illumination, probably approach- 

 ing the physical darkness which exists at 13 to 15 metres in the Lakes 

 of Auvergne, the striation of the valves of diatoms is less compact ; and, 

 mo reover, the general form of the frustules is more elongated and narroiv. 



2ndly. Under the influence of altitude, the striae are more numerous 

 and less robust. 



3. Raphe and Nodules. 



The raphe, which in Prof. H. L. Smith's classification (adopted in 

 this work) determines the primary divisions of the family of diatoms, 

 appears to be, at least on one portion of its length, a true cleft through 

 which the contents of the frustule are put into communication with the 

 liquid which surrounds it ( 3 ). 



The matter, however, appears to be placed beyond a doubt by the 

 researches of Mr. W. Prinz. This skilful microscopist has succeeded in 

 obtaining a perfect section of a large Navicula (which we believe to be 

 N. Dactylus) contained in the fossil deposit of Franzenbad. 



( j ) Die diatomeen der Hohen Tatra. Vienna, 1867. 



( 2 ) See De l'influence de la lumiere et de l'altitude sur la striation des valves des diatomees 

 par le Frere. Heribaud in Comptes-rendus. Acad. Sc., Paris (1894), cxviii., pp. 82-84; 

 J.R.M.S. (1894), p. 491. 



(3) Some authors, including Dr. A. Schmidt, deny that the raphe is a slit. 



