DIATOMACEOUS DEPOSIT FROM NORTH TOLSTA, LEWIS. 433 



Tribe II. PSEUDORAPHIDIE^E. 



Fam. 5. Fragilarie^e. 



25. Epithemia gibba, Klitz. 



Many of the specimens in the preparations consist of a single valve, rupture 

 of the connecting zone having taken place. The only notable circumstance in 

 which the valves differ from that figured by W. Smith (op. cit., vol. i. pi. i. figs. 

 13b., 13c), is in the character of the costse. These are less markedly arched 

 throughout ; in the central portion they are almost quite straight, but slight 

 arcuation appears towards the extremities, so disposed that the convexity of 

 the arches is directed away from the centre. 



Length, 01625 mm.; breadth of a single valve (zonal aspect) 0*0100 mm.; 

 breadth of entire frustule, 0*0200 mm.; breadth of narrowest part, 0*0162 mm.; 

 breadth at inflated extremities, 0*0175 mm. Strise, 8 in 0*01 mm. 



26. Epithemia gibba, Kiitz., var. rectimarginata, nov. (PI. XXIX. fig. 6). 



Valval view slightly plano-convex, one of the margins of the valve next 

 connecting zone straight. Dorsal inflation considerable, median, from its apex 

 the dorsal margin slopes outwards in a slightly concave line. Apices very 

 bluntly rounded, much inflexed on the side of the connecting zone. Costse 

 simple, irregular, straight or slightly arcuate at different parts of the valve. 



Length, 0*1500 mm. ; breadth of a single valve at median inflation, 0*0100 

 mm.; breadth half way between this inflation and the extremity, 0*0065 mm. ; 

 striae, 6 in 01 mm. 



This is a rare variety, and the valve figured was found isolated. It is at 

 once distinguished by its bluntly rounded extremities and by the irregularity 

 of its costee. A slight arcuation is found in these, especially on one side of the 

 frustule, near its centre, but they speedily become straight. Want of uniformity, 

 either in distance between the costse or in their direction, occurs at various 

 places, and these irregularities, though well marked, are not found at corre- 

 sponding points on both sides of the central transverse line. There is no 

 intercostal punctation, such as is figured in the varieties ptavallela and ventri- 

 cosum of Grunow. 



27. Epithemia argus (Ehrenb.), Kiitz., var. amphicephala, Grun. 



Specimens presenting a very considerable amount of affinity to this variety 

 of Grunow are occasionally found. They may be distinguished, however, from 

 the latter by the following characters : — (1) The convex margin of the valve is 

 much less flattened, and there is no slight inflation on any part of the concave 



