140 THE VOYAGE OF H.M.S. CHALLENGER 



central area which occupies about two-thirds of the diameter being rugged rather than 

 granulated. The intermediate zone has very fine radiating striae, while the external circle 

 is ornamented by regularly disposed granules arranged in a quincuncial manner. In this 

 form, as well as in that described by O'Meara, dark lines of variable length radiate 

 outwards from the perimeter of the central zone; these, however, are not "alternately 

 arranged," as referred to by O'Meara, but are distributed irregularly. Moreover, here 

 the margin, instead of being "finely striated," is granulated in a quincuncial manner, 

 each granule being distinctly defined with a magnifying power amounting to only 460 

 diameters. The additional circumstance, which is well seen in the present frustule, but is 

 not referred to by O'Meara, that the edge is marked by long, irregularly disposed dark 

 lines of variable thickness, is noteworthy. Although both Diatoms were collected in the 

 Antarctic Ocean, they were procured in localities separated by upwards of 40° of longitude; 

 yet the distinctions which have been pointed out cannot be looked upon as sufficient to 

 justify the establishment of two distinct species for the two interesting organisms. 



Hyalodiscus subtilis, Bail. var. japonica, nov. (Plate XVIII. fig. 4.) 



The specimen here shown was collected on the coast of the Sea of Japan. It agrees 

 generally with Hyalodiscus subtilis, Bail., 1 in the size of the umbilical area, in the 

 guillocke-like disposal of the granules, and in the character of the line of suture which 

 limits the central area. This line, however, in the present case is somewhat more indented, 

 while the granules are not difficult to detect and the margin is notably wider — character- 

 istics which are of sufficient importance to justify us in regarding this frustule as a 

 variety of Bailey's typical species. 



The fragment shown in Plate XXIV. fig. 4 probably belongs to the Hyalodiscus Icevis 

 of Ehrenberg. 2 



Oyclotella, Kg. 



The genus Oyclotella, which was instituted by Klitzing, differs but slightly from that of 

 Melosira — the frustules of the latter forming; longitudinal series, while those of the former 

 occur singly or at most united in pairs. Though Oyclotella may thus be regarded as but 

 another form of the genus Melosira, the retention of the former generic name is desirable, 

 as tending to prevent confusion, and to facilitate the record of so many distinct types. 

 Of the genus Oyclotella an excellent monograph has been prepared by the well-known 

 micrographer Dr Albert Grunow, and his work has been incorporated in the Synopsis des 

 Diatome'es de Belgique of Dr van Heurck. 



1 Pritchard, op. cit., p. 815, pi. v. fig. 60. 



2 Ehrenberg, Mikrogeologie, pi. xxxiii. 15, fig. 17 ; = Oyclotella Icevis, Kg., and allied to Cyclotella 

 physoplea, Pritchard, op. cit., p. 814. 



