DIATOMACEOUS DEPOSIT FROM NORTH TOLSTA, LEWIS. 429 



Entire frustule = 01025 mm. 



Length of distal inflation, 



. 00175 mm. 



„ median inflation, 



. 0-0600 „ 



„ proximal inflation, . 



. 0-0250 „ 



Breadth of distal inflation, 



. 0-0250 mm. 



„ „ constriction, 



. 0-0200 „ 



„ median inflation, 



. 0-0300 „ 



„ proximal constriction, 



. 0-0150 „ 



„ „ inflation, . 



. 0-0200 „ 



Number of strise in 0-01 mm., 6 ; the diameter of the puncta composing the strise does not 

 exceed - 0008 mm. 



Although this form cannot be assigned to the typical species, its peculiarities 

 are not of so divergent a character as to merit more than varietal distinction. 

 Rare. 



20. Gomphonema acuminatum, Ehrenb. (PL XXIX. fig. 5a). 



A considerable amount of divergence is found in the external form and 

 sculpturing of the numerous figures that have been given by Ehrenberg 

 (Mikt-ogeologie, pi. xv. figs. 87a, b) ; Kutzing [Die kieselschaligen Bacillarien 

 oder Diatomeen, pi. xiii. fig. 3); W. Smith {op. cit., vol. i. pi. xxviii. fig. 238); 

 Van Heurck [Synopsis des Diatomees de Belgique, pi. xxiii. fig. 16), &c, of this 

 diatom. The peculiarities noticeable in specimens from the present deposit are 

 the following : — (1) The cuneate valve is provided with three distinct marginal 

 inflations, the proximal, though least marked, beginning in the immediate 

 vicinity of the narrow extremity. In the case of the great distal inflation the 

 gradient is much greater towards the apical crest than on its proximal side. 

 In no case has asymmetry, such as is figured by Kutzing and Ralfs, been 

 observed. Although, as will be seen from the subjoined measurements of a 

 number of frustules, the ratio between the length of the three regions of the 

 valves varies somewhat, the distal is usually about one-half of the length of 

 the median. (2) The median bluntly conical apical crest has in most of the 

 specimens a height of about 0*0025 mm., and it has been noted that, although 

 the size of the frustule may vary very considerably, the height of this crest does 

 not change, although its transverse diameter at the base slightly increases. 

 Many of the specimens in the deposit correspond most closely to that figured 

 by Van Heurck, but in most of them the median and proximal inflations are 

 somewhat more pronounced than in that form, and the ratio between their 

 length and breadth do not agree, the Lewis specimens being relatively broader. 

 (3) The arrangement of the transverse striae in some of the specimens is some- 

 what different from that shown by W. Smith and Van Heurck, and agrees 

 more closely with that shown in Ralfs' and Kutzing's figures. In the distal 

 inflation the striae are in straight and not in arched lines, and at the greatest 

 transverse diameter of this part one runs at right angles to the tangent at its 



