DIATOMACEOTTS DEPOSIT FROM NORTH TOLSTA, LEWIS. 421 



Navicula macilenta, Grun.; Navicula tenella, Breb.; Navicula inoequistriata, 

 n. sp. ; Navicula interrupta (W. Sm.); Epithemia gibba, Kiitz., var. rectimar- 

 ginata, nov. ; Epithemia hyndmanii, W. Sm. ; Eunotia major (W. Sm.), Raben., 

 var. semelco?i$tricta, nov. ; Eunotia gracilis (Ehrenb.), Rab., var. semelmonticu- 

 lata, nov. ; Synedra ulna, Ehrenb., var. tolstensis, nov. ; Tabellaria fenestrata, 

 Kiitz. ; Tabellaria Jiocculosa, Kiitz. 



(5) Although not characterised by many new species, several interesting 

 and novel varieties occur, and various curious points of structure in connection 

 with forms already well known are clearly brought out. Among the new forms 

 the curious converse relationship in external form between Eunotia major, var. 

 semelconstricta and Eunotia gracilis, var. semelmonticulata, is noteworthy. 



(6) As shown by the measurements given below, the scarcely appreciable 

 change in the average fineness of punctation in some species {e.g., Gomplionema 

 acuminatum and Cyclotella antiqua — marginal circlet) notwithstanding the size 

 of the specimen is somewhat remarkable, although in the latter case it is often 

 associated with irregularity of sculpturing in other parts of the valves. 



Tribe I. RAPHIDIE^E. 

 Fam. 1. Cymbelle^e. 



1. Cymbella helvetica, Kiitz. 



Frequent. General form corresponding to that figured by W. Smith {Synop. 

 Brit. Diat., vol. i. pi. ii. fig. 24) ; but (1) the extremities are less acute, as shown 

 by A. Schmidt {Atlas d. Diat, pi. x. fig. 18) ; and (2) the striation of the valve 

 is not so arcuate. The striation differs from that on the frustule figured under 

 this name by Van Heueck {Synop. d. Diat. d. Belg., pi. ii. fig. 15), as no hya- 

 line area is found around the central nodule. 



Length, 0600 mm. ; greatest breadth at median inflation, 0137 mm. ; 

 striae, 10 in 0-01 mm. 



2. Encyonema ventricosum (Kiitz.), Grun. 



Rare. Some specimens agree very closely in form with that figured by W. 

 Smith {op. cit., vol. ii. pi. lv. fig. 346«) as Encyonema ccespitosum, Kiitz., but 

 the valval striation is disposed in a different manner. Instead of the lines 

 forming arches, with their convexities directed away from the median transverse 

 line of the frustule, the concavities are so disposed in the present case. The 

 specimens agree most closely with that figured by A. Schmidt {Atlas, pi. x. fig. 

 59). 



