46 THE VOYAGE OF H.M.S. CHALLENGER. 



Dimeregramma, Ralfs. 



Among filamentous Diatoms the genus Dimeregramma is closely allied to that of 

 Denticula, the only distinction consisting in the valves of the former always possessing a 

 smooth line which marks the longitudinal axis, while those of the latter do not exhibit 

 such a structure. 1 The majority of the known species which are now ascribed to 

 Dimeregramma, and which were discovered by Gregory before this genus was instituted, 

 were accordingly classed by him as belonging to the group Denticula, the forms recorded 

 by that observer, however, being marine, while the others were all freshwater species. 



Dimeregramma nanum, Greg., var. thaiticnsis, nov. (Plate XIX. fig. 5.) 



Among the Dimcregrammata collected by H.M.S. Challenger, the only noteworthy 

 specimens are those here figured. These greatly resemble the Dimeregramma nanum of 

 Gregory, 2 differing from it only in the form of the extremities, which in the present case 

 are more dilated and round. Such a distinction, however, cannot be viewed as possessing 

 more than a varietal importance. 



Terebraria, Greville. 



This genus was established by Greville in 1864, when he made us acquainted with the 

 Terebraria barbadensis, from the famous Barbados deposit in Cambridge. The definition 

 of the genus as given by him 3 ran as follows : — "Frustules in front view quadrangular, 

 binately conjoined, with transverse rows of conspicuous pseudopores and a longitudinal 

 serrated structure. Valve elliptical, with transverse rows of similar pseudopores." 



To the species made known by Greville, O'Meara added his Terebraria kerguelensis, 

 which was obtained from a gathering made by the Expedition in the vicinity of 

 Kerguelen, and which has been figured in the Journal of the Linnean Society (Botany) 

 vol. xv. pi. 1, fig. 4. I am, however, in doubt with regard to the correct determination of 

 this frustule. By referring to the description of the genus, or to the figure given by 

 Greville, the indented line of suture is found to be so evident as to constitute one of the 

 principal characteristics, whereas in the figure given by O'Meara no such lines of suture are 

 to be perceived ; but, on the contrary, on the zonal side the granules alternate, so that in 

 the line of division between the two adjacent valves there is an appearance resembling 

 that presented by an indented suturation. It would seem, therefore, from the fact that 

 O'Meara, in his definition of Terebraria kerguelensis, omitted to notice this circumstance, 

 that he took the central line of the zone in Terebraria barbadensis, Grev., not for what it 



1 Pritchard, op. cit., p. 790.] 



2 = Denticula nana, Greg., Diatoms of the Clyde, p. 23, pi. ii. fig. 34. 



3 Trans. Micros. Soc. Lond., new series, vol. xii. p. 8., figs. 12 and 13. 



