DIATOMACEOUS DEPOSIT FROM NORTH TOLSTA, LEWIS. 



441 



No. 



Diameter of Disc. 



Cellules on Valve. 



Marginal Puncta. 



1 



0-0200 mm. 



9 



10 in 0-01 mm. 



2 



0-0250 „ 



10 



10 





3 



0-0175 „ 



8 



10 





4 



0-0245 „ 



10 



10 





5 



0-0175 „ 



9 



8 





6 



0-0185 „ 



9 



8 





7 



0-0200 „ 



10 



10 





8 



0-0185 „ 



9 



10 





9 



0-0200 „ 



10 



10 





10 



0-0150 „ 



8 



10 





11 



0-0225 „ 



7 



8 





The following circumstances are noteworthy : — (1) The variability in the 

 number, size, and regularity of the valval cellules : in many cases those on one 

 side of the same valve are larger than those on the other. In some large valves 

 (No. 11) these cellules are larger and less abundant than in others (Nos. 2, 4) 

 of very nearly the same size. (2) The number of marginal puncta do not vary 

 greatly in their distance apart, whatever the size of the disc may be. 



EXPLANATION OF PLATE. 



Figs. 



la, lb 



Fig. 



2. 



Fig. 



3. 



Fig. 



4. 



Fig. 



4a. 



Fig. 



5. 



Fig. 



5a. 



Fig. 



6. 



Fig. 



7. 



Fig. 



8. 



Fig. 



9. 



Fig. 



10. 



Fig. 



11. 



Fig. 



12. 



Navicula obtusa, W. Sm., var. lata, nov. 



Navicula oblonga, "W. Sm., var. subparallela, nov. 



Navicula cardinalis (Ehrenb.), var. subconstricta, nov. 



Navicula inaequistriata, n. sp. 



Navicula macilenta, Grun., var. elliptica, nov. 



Gomphonema geminaturn, Ag., var. bipunctata, nov. 



Gomphonema acuminatum, Ehrenb. 



Epithemia gibba, Kiitz., var. rectimarginata, nov. 



Epitliemia argus (Ehrenb.), Kiitz., var. amphicephala(1), Grun. 



Eunotia major (W. Sm.), Raben., var. semelconstricta, nov. 



Eunotia gracilis (Ehrenb.), Raben., var. semeimonticulata, nov. 



Surirella nobilis, W. Sm. 



Surirella inaequisculpta, n. sp. 



Synedra ulna, Ehrenb., var. tolstensis, nov. 



All the figures are x 600. 



