32 p. T. cleve, synopsis of the naviculoid diatoms. 



6. D. mastogloidea Pant. (1886). — V. strongly constricted, with cuneate, obtuse ends. 

 L, 0,07 8; B. 0,0 24; at the constirction 0,oi3 mm. Marginal cellules about 10 in 0,oi mm. Cellules 

 of the valve 14 in 0,oi mm. — Nav. mast. Pant. I. p. 27 PL XXI f. 192. 



Marine: Hungary, fossil (Pant.). 



This species appears, to judge from the fig. in Pantocsek's work, to connect D. marginatq, 

 and D. jamaicensis • , ; 



7. D. rugosa Temp. a. Brun. (1889). — V. narrow, about 7 times longer than broad, gently 

 constricted, and with linear, subtruncate segments. L. 0,2 to 0,22.^; B. 0,026 to 0,03, at the con- 

 striction 0,013 mm. Terminal fissures in contrary directions. Marginal cellules small, 5 in 0,oi 

 mm. Cellules of the valve obscure. — Mastogloia rugosa Temp. a. Brun. D. fo3s. du Japon p. 39 

 PL IX f. 20. Dictyoneis rugosa Cl. Diatomiste I. p. 17. A. S. Atl. CLX f. 33. 



Marine: Japan, fossil (Temp. Br.). 



8. D. Pantocsekii Cl. (1890). — V. gently constricted, with tongue-shaped segments and 

 obtuse extremities. L. 0,u; B. 0,034, at the constriction 0,027 mm. Median line with the termi- 

 nal fissures in contrary directions and approximate central pores. Marginal cellules 5 in 0,oi mm., 

 of equal size, forming an uninterupted marginal band. Cellules of the valve about 17 in 0,oi mm., 

 arranged in irregularly undulating transverse and longitudinal rows. Transverse strige 25 in 0,oi 

 mm. — Navic. mastogloidea Pant. II PL XXVI f. 387. — Bid. Pant. Cl. Diatomiste I p. 16. 



Marine: Hungary, fossil! 



9. D. panduriformis Cl. (1881). — V. slightly constricted, with tongue-shaped segments 

 and obtuse ends. L. 0,097; B. 0,027, at the constriction O.019 mm. Terminal fissures in contrary 

 directions. Marginal cellules about 4 in 0,oi mm., af unequal size, absent in the middle and at 

 the ends. Transverse striae 20 to 21 in 0,oi mm. — Mastogloia panduriformis Cl. N. R. D. p. I 

 PL I f. 1. Diet. pand. Cl. Diatomiste I p. 16; 1890. 



Marine: Galapagos Islands! 



Pleurosigma W. Sm. (1852). 



Valve linear to lanceolate, more or less sigmoid, symmetrical. Median line sigmoid with 

 small central nodule and the ends turned in contrary directions. Axial area indistinct. Central 

 area indistinct or small. Structure: small puncta disposed in transverse and oblique rows. No lon- 

 gitudinal lines. Frustule with narrow, simple zone, arcuate or not. — Cell-contents (of P. angula- 

 tnm) two chromatophores, indented at their margins. The median part of each chrOmatophore 

 branches into a large elongated lobe along the interior of one of the valves, and into two similar 

 lobes on the other valve. Division begins by a fissure across the median lobe. After the division 

 of the cell the parts of the chromatophore migrate to the inner side of the old valves (0. MCller, 

 Ber. d. Deutsch. Bot. Ges. 1883 p. 478). 



The sigmoid Naviculee were named Navicula Sigma by Ehrbnberg. Hassall (A. History of 

 Brit. Freshw. Algse 1845 p. 435) proposed for them the name Gyrosigma, which was adopted by 

 Rabbnhorst (Die Slissw. Diat. 1853) but not by other diatomists, who preferred the newer name 

 Pleurosigma formed 1852 by W. Smith, who published the first monograph of the species (Ann. 

 Mag. Nat. Hist. 2 ser. IX p. 1). The genus Pleurosigma, as accepted by all diatomists, includes 

 forms with a structure of small puncta or alveoli, disposed in transverse rows, which are crossed 

 by other rows, either longitudinal, or obliquely decussating. There are no intermediate forms be. 

 tween these two types, and I think they may justly be regarded as different genera. For the 

 forms with the puncta in transverse and longitudinal rows, I adopt the name Gyrosigma, although, 



