74 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



DIMEREGRAMMA MINOR (Gregory) Ralfs 



(Van Heurck, Synopsis, pi. 36, figs. 10-lla; Gregory, Diat., Clyde, pi. 2, fig. 35.) 



DIMEREGRAMMA NANUM (Gregory) Ralfs 



(Pritchard, Inf., p. 790, pi. 4, fig. 33; Van Heurck, Synopsis, pi. 36, fig 11.) 



DIMEREGRAMMA OPULENS, new species 



Plate 15, figs. 6-7 



Valve elongated, naviculoid, constricted at the middle, then enlarged 

 and again constricted, whence the sides either run parallel to the 

 rounded apices or slightly approach each other; the entire valve, 

 except the apices, crossed transversely by massive moniliform costae, 

 divided into four parts by three narrow longitudinal lines, two of them 

 midway between the center and the sides, and one median line; each 

 of the four parts thus formed is two or three beads wide; apices ele- 

 vated above the rest of the valve, almost hyaline, but obscurely 

 marked with fine radiating beading. 



Length, 0.056-0.079; width, 0.014-0.017; costae, 5.5 in 0.01 mm. 



Type.— Cat. No. 43620, U.S.N.M. 



DIMEREGRAMMA PRISMATICUM, new species 



Plate 15, fig. 8 



Valve elliptical, prismatic, with acute apices; sides parallel at the 

 middle for one-third the length of the valve, then converging in straight 

 lines to the acute but rounded apices; valve transversely crossed by 

 heavy moniliform costae unequally divided into four parts by three 

 longitudinal and narrow lines, the median line being widest at the 

 center and narrowing toward the apices; the two outer or marginal 

 parts of the four divisions of the costal area one-half the width of the 

 two inner parts, each of the former ornamented with two beads and 

 each of the latter with four beads ; apical area small, triangular, ele- 

 vated above the rest of the valve, and hyaline. 



Length 0.045; width 0.017; costae 5 in 0.01 mm. 



Type.— Cat. No. 43621, U.S.N.M. 



Genus DITYLUM Bailey 



DITYLUM BRIGHTWELLII (West) Grunow 



(Van Heurck, Synopsis, pi. 114, figs. 3-9; Micro. Journ., 1860, pi. 8, figs. 

 1, 5, 8.) 



It is not advisable to change Bailey's spelling of the generic name 

 to Ditylium. 



Genus DONKINIA Ralfs 



For a discussion of the reasons for separating this and Rltoicosigma 

 from Pleurosigma see under those two genera. 



