REPORT ON THE DIATOMACEiE. 139 



of the forms at present only known as occurring in geological deposits may ultimately be 

 discovered in the living condition in the ocean. 



Porodiscus stolterfothii, n. sp. (Plate XII. fig. 8.) 



Forma rhombico-elliptica ; pseudo-ostiolo subrotundo mediocri ; striis radiantibus 

 stipatis. In Oceano Pacifico. 



This very singular Diatom at first sight recalls the Porodiscus oblongus of Greville, 1 

 which has been characterised as being an elliptico-oblong disc with a large pseudo-open- 

 ing ; this, by reference to the figure, being found to be equal to one-third of the transverse 

 axis and perfectly round. In the present case, however, the form of the valve is rhom- 

 boido-elliptical, and the pseudo-opening is much smaller and somewhat oval, while the 

 radiating lines of granules are much more crowded than in the Grevillean species. 



The association of these differences must be regarded as sufficient to constitute a new 

 species, which has been named in honour of the English Diatomist Dr H. Stolterfoth, 

 who has rendered much valuable aid in connection with the present work. 



Hyalodiscus, Ehrenb. 



The genus Hyalodiscus has been defined by Pritchard (History of the Infusoria, p. 

 814) in the following terms : — "Frustules simple, disciform; disc smooth, flat, its umbilical 

 portion or centre separated by a distinct suture ; " and he points out that it differs from 

 the genus Podosira, Ehrenb., in having a flat disc. I am, however, inclined to adopt the 

 view of Professor H. L. Smith, who thinks it very probable that the Hyalodisci are only 

 valves of Podosira — a genus which he unites to that of Mclosira, Ag. But on the other 

 hand, although Podosira has never yet been met with in long series of many frustules, 

 it seems to be desirable to retain it as a genus distinct from that of Mclosira, as we 

 thereby retain a distinction between two kindred series of forms. 



It may here be observed, on the other hand, that it is by no means easy to understand 

 how O'Meara described and figured, 2 under the name of Pyxidicula radiata, a form 

 collected by Mr Moseley at Kerguelen Island, as no Pyxidicula presents an umbilical 

 area which is separated by a line of suture from the rest of the surface of the valve. 

 O'Meara ought rather to have named his specimen Podosira (or Mclosira) radiata, as the 

 form of the valve, which is the only common character between his Diatom and the genus 

 Pyxidicula, is equally common to the two genera just named. 



Hyalodiscus (Pyxidicula, O'Me.), radiatus, var. nov. (Plate X. fig. 1.) 



A kindred, if not identical, frustule to that described and figured by O'Meara is here 

 represented. The disc has its surface divided into three well-defined zones, the wide 



1 Micr. Journ., n. s., voL iii. p. 65, pL iv. fig. 5. 



2 See Note on the Diatomaceous Gatherings made at Kerguelen's Land by H. N. Moseley, Journ. Linn. 

 Soc. Land. (Botany), vol. xv. plate i. fig. 9. 



