NEW FORMS OF MARINE DIATOMACE^). 537 



into two equal valves. The frustules are united by means of a circle of numer- 

 ous short, terminal processes of equal length, which ultimately separate in the 

 middle ; and the detached frustules then appear furnished at each end with a 

 beautiful coronet, or circle of miniature turrets. This mode of connection is peculiar, 

 although perhaps analogous to that in Biddulphia. In the subject of these 

 remarks, however, the connecting processes do not appear to be so distinctly a 

 continuation of the substance and structure of the body of the frustule, as are the 

 horns of that genus, and must rather be regarded in the light of appendages. 



With regard to the affinities of this beautiful little object, we may certainly 

 assume, that if a solitary frustule had alone been formed it would have been 

 referred by Kutzing at once to the genus Pyxidicida. But little or nothing is 

 known of the real nature of the variety of forms brought together under that 

 name. Eheenberg's own character for the original genus is as follows : — 



" Animal e familia Bacillariorum, liberum, lorica simplici, bivalvi (silicea) ; 

 solitarium, globosum ( = Gallionella divisione spontanea perfecta aut nulla). Die 

 Infusionsthierchen, p. 165. 



But Ehrenberg subsequently constituted other genera or subgenera to receive 

 the accumulating species ; and as the work in which they appear is not generally 

 accessible, I do not hesitate to give the characters verbatim in this place. 



" Dictyopyxis, nov. gen. Pyxidiculce generis ea bivalves subglobosse aut tur- 

 gidae formae, quae valvularum testae strictura simpliciter cellulosa insignis sunt ab 

 iis, quae continua et simplici membrana silicea includuntur, aut appendicibus 

 variis mstructae sunt, gravius differunt et facillime distinguuntur. Cellulosas 

 igitur in Dictyopyxidis subgenere colligendas senserim. P. cruciata, Cylind?*us, 

 hellenica et Lens huic subgeneri nunc inscribendae sunt." — Ehrenb. Bericht. der 

 Berl Akad., 1844. P. 262. 



" Stephanopyxis, nov. subgenus. Pyxidiculce generis bivalves turgid se aut 

 subglobosse formae, quse valvularum testae structura cellulosa insignes sunt et 

 denticulorum, aculeorum aut membranae coronam in media quavis valvula gerunt 

 in hoc Pyxidiculce subgenere colliguntur." — Ehrenb., I. c, 1844. P. 264. 



" Xanthopyxis, nov. subgen. Pyxidiculce subgenus bivalve turgidum sub- 

 globosum. Valvularum testae silicae continuse integerrimae nee cellulosae, super- 

 ficie hispida, setosa aut alata."— Ehrenb., I. c, 1844. P. 264. 



Kutzing, in his Species Algarum (1849), reunites the whole, giving twenty- 

 two species, all of which, except two, are fossil. The frustules, according to him, 

 are " non concatenata." Mr Creswell's Diatom is, therefore, by a most import- 

 ant character, excluded. Taking a simple frustule, and leaving the processes out 

 of view, it much resembles Dictyopyxis hellenica {Ehrenb. Microgeologie, tab. xx, 

 fig. 32), and also Stephanopyxis appendiculata of the same work, tab. xviii., fig. 4 ; 

 but that species has only a single tooth at each end, and is provided with a sort 

 of narrow zone or annulus. It resembles still more closely Stephanopyxis apicu- 



VOL. XXI. PART IV. 7 E 



