DIATOMACEOTJS DEPOSIT FROM NORTH TOLSTA, LEWIS. 437 



with gently recurved extremities, and the striae faintly radiating. Average 

 length, 01000 mm. ; greatest breadth at the middle, 0-0075 mm. ; at the slight 

 constriction at the origin of the recurved extremity the transverse diameter 

 is 0-0060 mm. Strise 10 in 0-01 mm. 



38. Eunotia gracilis (Ehrenb.), Raben, var. semelmonticulata, nov. 



(PL XXIX. fig. 9.) 



Rare. Valve lunately curved dorsally, with a median dorsal depression 

 opposite to the ventral inflation. Ventrally there is a characteristic large sub- 

 central protuberance with uniformly convex margin, but situated a little towards 

 one end of the frustule, thereby rendering it asymmetrical. Extremities 

 obtuse, slightly recurved. Striation uniform throughout, slightly radiating, 

 and at the extremities faintly arcuate, the convexities of the curves being 

 directed towards the centre. 



Length, 0-1325 mm. ; greatest breadth, 0-0100 mm. ; strise, 12 in 0-01 mm. 



39. Spnedra ulna, Ehrenb., var. tolstensis, nov. (PI. XXIX. fig. 12). 



Very rare. The specimen figured was broken across transversely at a dis- 

 tance of 0-1625 mm. from the extremity. Professor Van Heurck has figured 

 several varieties with knobbed extremities, but none of these agree with the 

 present form with respect to the form of the apices. The slightly diverging 

 character of the margins show very close affinities to the specimen figured by 

 Van Heurck on his pi. xxxviii. fig. 3 (op. cit.), and with this, too, the transverse 

 striation of the valve fully agrees, but the terminal knob is much less prominent, 

 its transverse diameter being 0-0045 mm , and the striation in the knob is not 

 oblique, but parallel, to that found in other parts of the valve. The transverse 

 diameter of the constriction separating the terminal knob from the remainder 

 of the valve is 0-0030 mm. The appearance of the terminal region of the 

 frustule somewhat resembles that of Synedra ulna, var. danica, Ktitz., but in 

 the present case the extremity is more rounded, and the constriction bounding 

 it of a more definite character. No central hyaline space occurs in the frag- 

 mentary frustule observed. Striae, 15 in 001 mm. 



40. Fragilaria mrescens, Ralfs, var. exigua, Grun. 



Not common. The specimens observed agree with those figured by Van 

 Heurck (op. cit, pi. xlv. fig. 3). Length, 0-0225 mm. ; breadth (valval aspect), 

 0'0035 mm. Striation very inconspicuous. 



