448 



DRURIDGEA. 



The genus Podosira only comprises living diatoms, or those found fossil 

 in deposits of marine origin. There are about 20 species, none of which 

 belong to our regions. The form represented in the text (fig. 171), is Podosira 

 Adriatica {Kuiz.) Cntn., which lives in the Adriatic. 



GENUS 113. DRURIDGEA DONK, 1861. 



Only differs from Podosira by its 

 elliptic valves and more compressed 

 subquadrangular frustules. 



Fig. 172. Druridgea geminata. 



This genus only comprises a single species, D. geminata Donk. (Q.J. M.S., 

 1861, i., n.s., p. 13, pi. 1, f. 15 ; H.V.H. Atl., pi. 91, f. 25, 26*), represented 

 in the text (fig. 172), and which has been found in various localities in England: 

 Cresswell Sands (W. Arnott !) ; Tynemouth, Northumberland ; Druridge Bay, 

 Yarmouth Sands. We have never met with it on the Belgian shores, but it 

 will probably be found there. It is always found on the sand of the sea- 

 shore in frustules united in twos. 



GENUS 114. HYALODISCUS EHR., 1854. 



Valve orbicular, with a very dis- 

 tinct and finely-marked umbilicus, 

 furnished with rays or decussating 

 lines. 



Fig. 173. Hyalodiscus Stelliger. 



The Hyalodiscus are not essentially different from Podosira except in the 

 umbilicus, which is more or less distinct according to the species. The genus 

 includes about half a dozen very characteristic species, some living, some 

 fossil, only two of which inhabit our regions : H. Stelliger Bail. (fig. 173), and 

 H. subtilts Bail. var. Scoiica. 



