RADIOPALMA. 



57 



'@G 



Valve circular, convex, with 

 umbilicus raised, projecting, con- 

 solidated internally by a row of 

 robust costal united to one another 

 by a marginal lamina and a central 

 circular plate, pierced and raised 

 in the shape of a funnel at the 

 umbilicus, and sometimes also by 

 costas more delicate, concentric and 

 transverse to the first mentioned. 

 Valve, properly speaking, formed 

 of two plates (?) one robust, show- 

 ing large thinnings in the form of 

 irregularly quadrate holes, and 

 forming concentric rows ; the other 



showing fine granules, mostly corresponding to the thinnings of the 



other lamina. 



The Arachnoidiscus are amongst the most interesting diatoms known. 

 Nine species have been described, many of which appear to us to be only 

 varieties. The most characteristic and best known are A. ornatus Ehr., and 

 A. Ehrenbergii Bail, and Hat v., which are figured in the text (figs. 255 and 

 256), the general outline and structure being taken from my photographs. 

 The latter Arachnoidiscus has erroneously been stated by William Smith as 

 belonging to England. All the Arachnoidiscus found are either fossil or from 

 the tropical seas. 



GENUS 172.-RADIOPALMA BRUN., 1891. 



Fig. 256. 



Arachnoidiscus Ehrenbergii. 



Valve sub-circular, very 

 delicate, with hexagonal 

 striation interrupted by plaits 

 more or less marginal, often 

 divided dichotomouslv. 



Fig- 257. Radiopalma dichotoma- 



