DIATOMACEOUS DEPOSIT FROM NORTH TOLSTA, LEWIS. 427 



16. Navicula interrupta (W. Sm.). 

 = Pinnularia interrupta, W. Sm. 



Very rare. The character of the ornamentation agrees essentially with that 

 of the frustule figured by W. Smith (op. tit., vol. i. pi. xix. fig. 184), the central 

 hyaline space being somewhat infundibulate at both sides, but it differs from 

 Schmidt's specimen (Atlas, pi. xlv. fig. 72) in the entire absence of a diamond- 

 shaped median hyaline area. The outline of the central part of the valve is 

 somewhat more convex than is indicated by Smith, or than the corresponding 

 part of the Navicula hilseana, Janisch, which is very closely allied to it, but less 

 so than that of Navicula brauniana, Grun., to which it also bears great external 

 similarity. The appearance of the capitate terminal knob distinguishes it from 

 Navicula appendiculata var. irrorata, Grun., although the character of the 

 central hyaline region is very much the same in both. 



Length, 0*0600 mm.; greatest breadth, - 0105 mm.; transverse diameter of 

 terminal nodules, 0*0060 mm. 



The specimen observed was extremely hyaline, the valval sculpturing being 

 only clearly recognised after considerable difficulty. 



17. Stauroneis phosnicenteron, Ehrenb. 



Specimens sometimes somewhat damaged. Striation exceedingly delicate. 

 Length, 01525 mm.; greatest breadth, 0*0250 mm.; breadth of transverse 

 stauros, 0*0040 mm. 



18. Stauroneis anceps, Ehrenb. 



Rare. General form of valve typical. Stauros not hour-glass shaped, but 

 with parallel margins, and running outwards to the margins of the valves. 

 Length, 0*0800 mm.; greatest breadth, 0*0175 mm.; transverse diameter of 

 terminal knob, 0*0050 mm.; transverse diameter of stauros, 0*0025 mm. Stria- 

 tion exceedingly delicate. 



Fam. 3. Gomphoneme^e. 

 19. Gomphonema geminatum, Ag., var. bipunctata, nov. (PI. XXIX. fig. 5). 



Valves inflated at the centre and at both extremities, the outlines of all the 

 inflations being less acute than in the typical species. This frustule differs in 

 the following respects from that figured by W. Smith :* — (1) If the wider 

 extremity be called distal, and the narrower, to which the stipes should be 



* Synopsis British Diatomacece, vol. i. pi. xxxvii. fig. 235, 1853. 

 VOL; XXXIII. PART II. 3 R 



