148 THE VOYAGE OF H.M.S. CHALLENGER 



Euodia (Bailey), Cstr. 



Among the Auguliferese, Pritchard 1 enumerates the genera Euodia, Bail., and Hemi- 

 discus, Wall. The definition of the latter, as given by Wallich, 2 is as follows: — "Frustule 

 free ; valve arcuate, with a marginal nodule ; cellulation hexagonal, radiate." Professor 

 H. L. Smith, however, according to Dr van Heurck, 3 speaks of Hemidiscus as possessing 

 somewhat different characters, namely, " valves celluleuses, centre blanc, marge veinee." 



The genus Euodia, Bail., is defined by Pritchard in these words — "Frustules cellulose 

 or granulate, in lateral view lunate ; " while Professor H. L. Smith, 4 after describing the 

 Palmeria of Greville thus — " Valves with indistinct umbilicus, finely punctate with 

 radiating lines, dorsal and ventral margins with minute teeth or spines " — characterises 

 Euodia as follows — " All others, dorsal margin without spines, ventral frequently with a 

 small pseudo-nodule." 



Thus in the case of Euodia Bailey only requires the lunate form and the cellulose 

 structure of the valve as distinguishing features, while Smith demands that the dorsal 

 margin be devoid of spines and the ventral often provided with a pseudo-nodule. In the 

 case of Hemidiscus, on the other hand, Wallich regards an arcuate form, a cellular radi- 

 ating structure, and a marginal nodule as essential ; whereas Professor H. L. Smith 4 

 insists on " cellular valves, clear centre, and veined margin." 



To avoid such confusion, I believe, from the observations which I have been enabled 

 to make, that the two genera, Hemidiscus and Euodia, should be united into one. This 

 view has also been propounded by Pritchard, who, after defining the Hemidiscus of 

 Wallich, adds — " We doubt whether the Hemidiscus be distinct from Euodia, since the 

 only distinction seems to be the marginal nodule of the former — a character perhaps 

 overlooked by Professor Bailey." 



In the marine soundings of the Challenger the lunate forms of Euodia and Hemi- 

 discus have to be recorded as among the least uncommon forms of Diatoms. They 

 are found of all sizes and in all positions, and they are more or less arcuate in 

 outline, having circular or parabolic curves, the ventral line being almost plain or 

 possessing an inflation in the centre, and being not much less swollen than the dorsal line. 

 I have frequently found specimens with a more or less distinct ventral nodule, but have 

 at the same time observed a few devoid of any such structure. The latter I named 

 Euodia, and the former Hemidiscus ; but on several occasions I had the opportunity of 

 observing some semi-lunate valves in series of two or more frustules, and on examining 



1 Pritchard, op. cii., p. 852. 



2 On the Siliceous Organisms found in the Digestive Cavities of the Salpaj, and their relation to the Flint 

 Nodules of the Chalk Formation, Micr. Journ., vol. viii. p. 42, 1860. 



3 Le Microscope, sa construction, son maniement et son application special a l'anatomie vegetale et aux 

 diatomees, par le Dr Henri van Heurck. — Troisieme edition, Bruxelles, 1878. 



4 The Lens, vol. i. p. 18, 1872. 



