682 Transactions of the Society. 



Yar. consimilisGrvin. Length, •4mn). ; breadth, "01 3 mm.; 

 keel puncta, 5-7 ; transverse striae, 23-26 in '01 mm. 

 This form stands midway in the transition from N. Sigma 

 proper. 



Var. genuina Grmi. Length, •25 mm.; breadth, "Ollmm. ; 



keel puncta, 7-9 ; transverse stria3, 20-24 in '01 mm. 

 Var. ? fasciculata Grun. Lengtli, • 095 mm. ; breadth, 

 •036 mm.; keel puncta, 4-6; transverse striae, 28-30 in 

 •01mm. It is the Homeoeladia signioidea'W. Sm., at 

 least what I have hitherto considered to be that species. 

 I cannot admit that it is a Homeoeladia. It is dis- 

 tinguished from N. Sir/ma proper by its distant keel 

 puncta as well as by its delicate striation. 

 Var. intercedens Grun. Length, -30 mm.; breadth, 

 •009 mm.; keel puncta, 6-7; transverse striae, 28-33 in 

 •01 mm. 

 Var. ? Sigmatella Gregory. Length, 32 mm. ; breadth, 

 •006 mm. ; keel puncta, 8-10 ; transverse striae, 25-28 in 

 •01 mm. Valves more or less strongly sigmoid. 

 [Nitzschia Sigmatella Gregory is no Nitzschia. Wm. Smith also 

 refers this form to the genus Nitzschia under the sp. name oicurvula. 

 Dr. Lewis considersit an extreme form of Surhella, and names it 

 ;S'. intermedia. I have seen authentic specimens of N. Sigmatella 

 and i\'. curvula W. Sm., and am satisfied that they are identical, 

 and also that they are not allied to Nitzschia excepting in 

 outhne.— F. K.] 



Var. ? rigida Grun. Length, ■ 20 mm. ; breadth, • 008 ; keel 

 puncta, 8-11 ; transverse strise, 28-32 in "01 mm. Ac- 

 cording to Dr. Arnott this is Am])liiijleura sigmoidea 

 W. Sm. {A. rigida Kg. ?). A small form of this is 

 perhaps Navicula lamprocampa Ehr. in Kg. B. Nitzschia 

 Jtexa Schumann appears to belong here. 

 Var. anguilla Schumann. Valves acute; 0^045 mm. in 

 length; • 003 mm. in breadth; keel puncta, 11-12; 

 transverse striae, 29-30 in "01 mm. 

 Var. ? Clausii Hantzsch. Resembles the preceding. Valves 

 with produced rounded apices, "015 mm. long, •004 mm. 

 broad; keel puncta, 10-11 in •Ol mm.; striae very 

 delicate. 

 In this series (in which the distinctions are based upon the 

 size of valve, and distance of keel puncta and striae) may be 

 included some irregular forms. 



Var. ? suhrecta Grun. Valve lanceolate sigmoid, flexure very 

 slight, often scarcely distinguishable. Length, • 1 1 mm. ; 

 breadth, '008 mm. ; keel puncta, 10-11 ; transverse striae, 

 28-30 in -01 mm. 



