94 TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 



The nodules usually appear in the form of round or elliptical pro- 

 minences. The central nodule is sometimes sub-quadrangular, and the 

 terminal nodules are sometimes produced into the form of a crochet 

 hook (Ex. Cymbella Helvetica, pi. i, fig. 43). 



When the central nodule is considerably enlarged transversely it receives 

 the name of Stauros. 



% 4. Endochrome. 



We have already examined the chemical and spectroscopical characters 

 of the endochrome. It still remains to consider it as a criterion in 

 certain classifications. The endochrome, as previously mentioned, is met 

 with in two different states, viz., as plates or granules. These two 

 states determine the primary divisions into sub-families in the 

 classification of diatoms adopted by Professor Pfitzer, who has published 

 an important work on the organography and evolution of diatoms. (') 



Afterwards the classification was taken up and completed by the 

 examination of different marine forms by Mr. Paul Petit. Although 

 we have not adopted the classification based on the endochrome, we 

 give in the two pages following the key of the system as published by 

 Mr. Petit in 1892. ( 2 ) 



The table gives a resume of our present knowledge of the different 

 forms and positions of the endochrome. 



The diatomist should, of course, be perfectly familiar with the 

 modifications of the endochrome if he wishes to avoid finding himself 

 in the position of a naturalist who contents himself with merely 

 examining the skeletons of those organisms whose study he wishes 

 to master completely. ( 3 ) 



But one should not however exaggerate the value of the characteristics 

 drawn from the modifications of the endochrome, and it is nowadays 

 generally accepted that these modifications cannot claim for classifica- 

 tion purposes the importance which Mr. Paul Petit has desired to 

 attribute to them. With the exception of the primary divisions into 

 Placcochromea; and Coccochromeae it is necessary in every instance 

 to recur to the characteristics given by the valves in order to establish 

 families. 



( ) See the chapter " Bibliography." 



( 2 ) In the Journal de Micrographie of Pelletan. 



( 3 ) Recent observations have shewn that the form of the endochrome is not as invariable 

 in the same genus as it was believed to be ; thus, e.g. , in different Navicula, N. EllipHca, etc., 

 the endochrome has been observed in a granular state. Count Castracane {Brtlbissonia, 1878, 

 p. 75), asserts that the arrangement of the endochrome is variable, and that the arrangement 

 of this substance " in small masses or distinct globules and of the same size, is the prelude 

 to reproduction." 



