DIATOMACEOUS DEPOSIT FROM NORTH TOLSTA, LEWIS. 425 



character of the terminal areola?, though the want of symmetry between the 

 two sides of the valve is very much less marked. 



Length, 0*1625 mm.; greatest breadth, 0*0235 mm.; diameter of terminal 

 areola, 0-0040 mm.; striae, 6 in 0*01 mm. 



11. Navicula oblonga (W. Sm.), var. subparallela, nov. (PI. XXIX. fig. 2). 



Margins of valve almost straight, and tapering but very slightly to the 

 extremities. Apices bluntly rounded. Terminal areolae ovate, and placed very 

 near the extremities ; central hyaline space elliptical. Hyaline areas bordering 

 the raphe lentelliptical. Costae disposed as in the type in the central region, 

 but altogether different at the extremities, being directed very obliquely out- 

 wards and towards the central transverse axis of the valve for a space of 



0125 mm. 



Length, 0*0900 mm.; greatest breadth, 0*0100 mm.; diameter of terminal 

 areolae, 0*0028 mm. Striae 10 to 12 in 0*01 mm. 



This Diatom agrees in form with the Navicula oblonga figured by A. Schmidt 

 {op. cit., pi. xlvii. fig. 67), but the details of its ornamentation are altogether 

 different, especially with respect to the terminal costae and the form of the 

 median hyaline area. 



12. Navicula tenella, Breb. 



A few specimens occur in the deposit which agree essentially with the 

 forms figured by Van Heueck (op. cit., pi. vii. figs. 21, 22). The striation is 

 straight and radiating, the median raphe is well defined, and the central clear 

 space an ellipse of great excentricity, but the extremities are somewhat more 

 obtuse, and the somewhat larger terminal areolae are situated just beneath the 

 margin. 



Length, 0*0500 mm. ; greatest breadth, 0*0100 mm. ; striae, 8 to 10 in 



001 mm. 



13. Navicula cardinalis (Ehrenb.). 

 = Pinnularia cardinalis, Ehrenb. 



Several specimens. Some of the forms may be distinguished from that 

 figured by W. Smith (op. cit, pi. xix. fig. 166) (1) by the somewhat greater 

 swelling at the centre of the frustule ; (2) by the occurrence of hyaline areas 

 around the median raphe, which are either bounded by straight lines externally, 

 or are only very slightly elliptical ; (3) by the presence of a less regularly con- 

 toured hyaline central area ; and (4) by the more rounded appearance of the 

 terminal areolae. 



