DIATOMACEOTJS DEPOSIT FROM NORTH TOLSTA, LEWIS. 



439 



canaliculi is well pronounced, though much less marked than in S. valida, A. S., 

 but not unlike that in many forms of S. bifrons, Kiitz. 



Some perfect specimens of large dimensions have the outlines of the valves 

 not convex throughout its entire extent, as normally occurs, but very slightly 

 concave in the central area. One such specimen examined showed the follow- 

 ing dimensions : — Length, 0-2700 mm. ; breadth at the widest part, 0*0725 mm. ; 

 transverse diameter at the concave area, 0*0550 mm. Generally the flattening 

 of the convexity of the margins advances as the frustules increase in length. 

 This becomes clear from a comparison of the unclernoted measurements of 

 frustules occurring in the deposit. It is further to be noted that, notwith- 

 standing this curious relationship, the relative and absolute sizes of the canali- 

 culi do not alter at all, or only within very narrow limits, and in the middle 

 zone, having a slightly larger diameter in larger forms, the average distance 

 between the outer extremity of one and that of its immediate neighbour being 

 0*0125 mm. in large specimens belonging to group A (see infra), and 0*0155 mm. 

 in those belonging to group B. 



No. 



Length of Specimen. 



Breadth of Specimen. 



Ratio. 





' 1 



0-3050 mm. 



0-0725 mm. 



4-206 : 1 



A. 



2 



0-3500 „ 



0-0800 „ 



4-375 : 1 



Larger forms, < 



3 



0-3000 „ 



0-0750 „ 



4- :1 





4 



0-3125 „ 



0-0700 „ 



4-464:1 





5 



0-3050 „ 



0-0700 „ 



4-357:1 





' 6 



0-1750 „ 



0-0675 „ 



2-592:1 



B. 



7 



0-1775 „ 



0-0650 „ 



2-73 :1 



Smaller forms, -\ 



8 



0-1850 „ 



0-0700 „ 



2-642 : 1 





9 



01800 „ 



0-0750 „ 



2-4 :1 





10 



0-1775 „ 



00725 „ 



2-448 : 1 



In acid preparations detached rings of this species are very common. In 

 some specimens observed the alse assumed a somewhat panduriform shape, and 

 were perfectly symmetrical on both sides of the valves, but careful examina- 

 tion with high powers showed that the appearance was due to the rupture of 

 the alse from the valve, and to their subsequent inflexion towards one another 

 upon the surface. 



