132 THE VOYAGE OF EM.S. CHALLENGER. 



bear at their base, and at a considerable distance from the margin, a denticule which 

 serves to distinguish the present Diatom from other species. The apices of the non- 

 denticulated compartments are rounded, so that a central hyaline hexagonal area results, 

 which is circumscribed by straight and concave sides. The margin of the valve is 

 convex. 



The specific name has been given in honour of Mr Shrubsole, who is well known for 

 his researches on the fossil Diatoms of the Lower Eocene of the London basin. 



Asteromphalus, Ehrenb. 1 



This genus essentially consists, like that of Asterolampra, Ehrenb., 2 of an areolated 

 granulated disc, which bears a hyaline star and umbilical lines of division. It differs, 

 however, from the latter in the single important circumstance that one of the radial areas 

 is so much narrower than the others as to be almost obliterated. The umbilical lines and 

 the form of the granulated segments constitute the principal distinctive characteristics 

 between the specific types. The genus, which was established by Ehrenberg, has been 

 defined as follows : — " Frustules simple, disciform ; disc as in Asterolampra, but with 

 two of the punctated compartments approximate, and the interposed ray narrower than 

 the others." 3 



Marine Diatoms belonging to this group are by no means rare, but the transparency 

 of the valves is such that they are often not perceived in Canada Balsam preparations. 

 Although the genus Asteromphalus is richer in species than that of Asterolampra, the 

 following new forms belongino- to the former have been observed in the Challenger 

 collections : — 



Asteromphalus ovatus, n. sp. (Plate V. fig. 7.) 



Elliptico-rotun datum ; segmentis rotundato-complanatis ; lineis umbilicalibus rectis vel 

 non angulariter curvatis, ab area pyriformi dimanantibus ; areis radialibus marginem non 

 attingentibus. In mari Antarctico. 



This very small oval Diatom, which in the figure is magnified 1000 diameters, is dis- 

 tinguished by the circumstance that the umbilical lines do not separate from one another, 

 as in the others, at a central point, but originate from the circumference of the area of the 

 obliterated radius. This characteristic has been found, it is true, in other allied forms, 

 among which Brebisson constituted the genus Spatangidium ; 4 but this genus has not 



1 Monatsber. d. h. Akad. d. Wiss. Berlin, 1844, p. 198. 



2 See definition in Monatsber. d, k. Akad. d. Wiss. Berlin, 1844, p. 73. 



3 Pritchard, op. ait., p. 836. 



4 e.g., Spatangidium flabrllatum, Spatangidium peltatum, Spatangidium araclme, Spatangidium heptactis, in 

 Bull. Soc. Linn, de Normand., voL iii. ; Spatangidium ralfsianum, Norm., Greville on Diatomacete observed in 

 Californiai) Guano, Micr. Journ., vol. vii. p. 161. 



