43 6 



THALASSIOSIRA. 



var. delicatula H.V.H., resembles somewhat ' the preceding 

 variety, but differs from it by the margin in girdle view being narrower, 

 and by the valve having only short simple spines at the central portion, 

 and furnished on the margin with a row of long, slender, recumbent 

 spines. Silica delicate ; puncta scattered, conspicuous ; costae feeble, 

 distant. 



Fossil. Sta. Monica (Weissflog, in prep., No. on). 



GENUS 105 THALASSIOSIRA CLEVE, 1872. 



Frustules more or less distant, united by a 

 mucous delicate filament ; valves round, 

 slightly silicious, with striation radiant, dicho- 

 tomous, furnished with small submarginal 

 spines. 



Fig. 161. 



Thalassiosira Nordemkioldii, This genus only includes R. Nordenskioldii 



(fig. 161), a delicate species, whose valves are from 2 to 2-5 c.d.m. in diameter, 

 and which is found floating on the surface of the Arctic seas in laige quantities. 



GENUS 106. SKELETONEMA GREV, 1865. 



Frustules cylindrical, united into a filament ; 

 valves inflated, heuri-spherical or digitati- 

 form, with a marginal corona of long teeth 

 or spines, simple or bifurcate at the summit. 



The genus Skeletonema includes about a dozen 

 species, of which only a single species belongs to 

 our region ; it is the : 



Fig. 162. 

 Skeletonema mirabile. 



