160 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



As there is only one figure of this species published, and in a paper 

 not easily obtained, I give a photograph here. To those who see an 

 advantage in Grunow's creation of the genus Ardissonia, for these 

 robust Synedral forms with hyaline side lines, this species would be 

 so classified. 



SYNEDRA ROBUSTA Ralfs 



(Van Heurck, Synopsis, pi. 42, figs. 6-7; Pritchard, Inf., pi. 8, fig. 3; Peragallo, 

 Diat., France, pi. 78, figs. 3-5.) 



This is not easily separable from S. formosa Hantzsch. 



SYNEDRA UNDULATA (Bailey) W. Smith 



(Gregory, Diat., Clyde, pi. 14, fig. 107; Van Heurck, Synopsis, pi. 42, fig. 2; 

 Peragallo, Diat., France, pi. 78, fig. 7.) 



Genus SYRINGIDIUM Ehrenberg 



SYRINGIDUM DAEMON Greville 



(Micro. Journ., 1866, p. 83, pi. 9, figs. 22-28.) 



This species and S. americanum L. W. Bailey (Van Heurck, Syn- 

 opsis, pi. 106, fig. 2) are closely alike; the latter was named in 1861 

 (Bost. Journ. Nat. Hist., pi. 2, figs. 62-64). 



Genus TERPSINOE Ehrenberg 



TERPSINOE INTERMEDIA Grunow 



(Schmidt, Atlas, pi. 199, figs. 1-8.) 



TERPSINOE MUSICA Ehrenberg 



(Schmidt, Atlas, pi. 199, figs. 9-13.) 



Genus THALASSIOTHRIX Cleve and Grunow 



THALASSIOTHRIX FRAUENFELDII Grunow 



(Van Heurck, Synopsis, pi. 37, figs. 12-15; Gran, Nord. Plankt., p. 117, fig. 

 159.) 



This strictly plankton diatom was only accidentally met with in 

 these dredgings. 



TRIBRACHIA, new genus 



Diatoms growing in chains, each member joined to the next by 

 means of three long, spirally twisted arms; in side (girdle) view each 

 frustule is seen to be a cylinder, its length about three times its width, 

 the ends (valves) being convex; from the margin of each valve there 

 arise vertically three massive, hyaline arms, sharply bowed inward 

 at the middle, so that they interlace, and also having a slightly spiral 

 twist; at their extremities they are joined to the three corresponding 

 arms of the next frustule, not end to end but by lateral contact; 

 their length is somewhat above one-half that of the cylindrical frus- 

 tule, so that each cylinder in the chain is widely separated from 



