I04 Clarence J. Elmore 



2. Brebissonia interposita (Lewis) Kuntze. Rev. Gen. Plant., 

 Ill, 2, p. jp8. ipoo. 

 A^az'icula interposita Lewis. Proc. Acad. Phil., p. 15, pi. 2, 



fig. 19. 1865. 

 Frustulia interposita (Lewis) CI. Syn. Nav. Dial., I, p. I2j. 



1894. 

 PI. 13, figs. 510-511- 



Linear-elliptical, 82-130 /i, long, 12-27 /^ wide, apices rounded; 

 central nodule elongated, terminal nodules round, close to the 

 ends; transverse striations 20-24 in 10 /x,. perpendicular to the 

 raphe, longitudinal striations about 18 in 10 fx. 

 Fossil : Agate. 



26. Gyrosigma (Plcurosignia) 



Gyrosigma Hass. Brit. Freshzv. Alg., p. 43^. 1845. 



Pleurosigma W. Sm. in Ann. Nat. Hist., II, Vol. p, p. 5. 18^3; 

 Achnanthosignm Reinh. ; Endosigma Breb. ; Scalprum Corda; 

 Staurosigma Grun. 



Etym.. from Gr. yvpo<;, curve, and o-ty/Aa, the letter s, in allusion 

 to the curved form of the cells. 



Cells single, free or rarely enclosed in gelatinous tubes ; straight 

 and oval-linear in girdle view, sigmoid in valve view; valves bi- 

 laterally symmetrical, sigmoid-lanceolate ; raphe median, sigmoid; 

 central nodule small ; striations crossing in three directions (decus- 

 sate), or at right angles (rectangular), reaching almost to the 

 raphe. Chromatophores two large widely overlapping plates lying 

 next to the girdle. 



A. Striations running in three directions. I. G. dcUcaluhtm. 



AA. Striations running in two directions. 



b. Transverse and longitudinal striations equally distant. 



2. G. aciiniiiiatjiiu. 

 bb. Transverse striations more distant than longitudinal. 

 c. Lanceolate. 



d. Ends acute. 3- G- kutsingii. 



dd. Ends acuminate. 4- G. parJ^crii. 



cc. Linear. 



d. Tapering toward the ends. 5. G. spenccrii. 



dd. Ends abrupt'y and obliquely rounded. 6. G. scalproidcs. 

 bbb. Longitudinal striations more distant than transverse. 



7. G. aticnuatiim. 



