ACANTHODISCUS. 



5*9 



Sect. III. Acanthodisce^;. Characters of the genus. 

 GENUS 182. ACANTHODISCUS PANT, 1892, in litt. 



Valve convex, with coarse scattered puncta throughout, between 

 the centre and the margin a rugose, dentate ridge, more or less 

 interrupted or plicate. 



Fig. 272. Acanihodiscus Clypeolus 



In his " Diatomees nouvelles, 1891," Professor Brun established a genus 

 called Cotyledon, based especially on the presence of a ridge. Prof. De Toni 

 has suppressed this genus because a genus of the same name already existed. 

 This does not appear to me to be a sufficient reason, as there can be no 

 confusion possible between a phanerogamic genus and one of diatoms. 



But the genus of Mr. Brun, as the learned diatomist himself subsequently 

 stated, was only given as a landmark, and it includes very different forms. 



I therefore think it is more convenient to adopt the genus which Dr. 

 Pantocsek has communicated to me, and to which he gives the name of 

 Acanthodiscus, and which is founded as much on the striation as the presence 

 of the ridge. 



The genus Acanthodiscus therefore includes two species, A. clypeolus 

 (Brun.), a variety of which will be found figured in the text (fig. 272), drawn 

 from one of my photographs, and A. rugosus Pant. Both have been found 

 fossil at Kusnetzk, Russia. 



As to the curious form which Mr. Brun calls Cotyledon coronalis 

 it requires further examination, and may constitute a new genus, Brunictta 

 H. V.//., in which may also perhaps be included his C. circutaris. 



