916 



SYSTEMATIC HISTOEY OF THE INFUSOEIA. 



straight), with a sigmoid median line, central and terminal nodules, and fine 

 decnssating striae, which are resolvable into dots. Plenrosigma is distinguished 

 from Donkinia and Amphiprora by the elongated-narrow front view and 

 more depressed valves. The median line, also, is sigmoid, whilst in those 

 genera it is usually straight, or appears sigmoid merely from the twisting of 

 the frustule. This genus was first separated by Dr. Hassall from N'avicula, 

 under the name of Gyrosigma. Hassall, however, Hke Ehi-enberg, erroneously 

 considered it identical with the Sigmatella of Kiitzing, whereas they belong 

 to very distinct families ; and even their sigmoid forms belong to difi'erent 

 surfaces, Sigmatella having it in the front, and this genus in the lateral view. 

 Gyrosigma has been adopted by Eabenhorst and others ; nor do we think the 

 name so objectionable as to render its rejection necessary. If, then, we admit 

 Professor Smith's name, we do so for the reasons given by Brebisson : — 

 *' Gyrosigma (Hass.). — Peutetre ce dernier nom de genre n'etait-il pas bien 

 convenable selon les lois de la nomenclature ; dans tous les cas, il est certain 

 que, malgre son droit de priorite, il est a pen pres generalement abandonne. 

 D'ailleurs, on est d'autant moins dispose a reprocher ce changement de nom 

 a M. W. Smith, que le soin tout monographique qu'il a apporte a I'etude des 

 nombreuses especes de Plenrosigma qu'il a decouvertes, en fait un genre tout 

 a lui" (BrebDC. p. 17). Ehrenberg does not admit Plenrosigma, because it 

 ^' does not difier in its physiological characters from Navicula " (EB. 1854, 

 p. 236). 



* Frustules rostrate. 2 f Beaks short, stout. 



t Beaks filiform. 



Fi.BvrROSiGMA Fasciola (E., S.). — Tur- 

 gid-lanceolate, with long linear beaks 

 abruptly curved in contrary directions ; 

 stria3 64 in -001", indistinct. SD. i. p. 67, 

 pi. 21. £ 211. = Ceratoneis Fasciola, E, 

 Marine. Europe, (xii. 60, 61.) Colour of 

 dry valve pale-pink. 1-430". Mr.Somtt 

 states this Diatom near Hull is very 

 small, the markings 90 in -001", while 

 those from Boston in Lincolnshire are 

 large, with only 50 striae in -001". 



P. macrum (S.). — Elongated slender- 

 lanceolate, with very long filiform beaks 

 curv^ed in contrary directions ; transverse 

 strise 85 in -001", very indistinct. SD. 

 i. p. 67, pi. 31. £ 276.*^ Brackish water. 

 England. 



P. prolongatiim (S.). — Narrow-lance- 

 olate, gradually tapering into slender 

 beaks curved in contrary directions ; 

 transverse striae 65 in -001", indistinct. 

 SD. i. p. 67, pi. 21. £ 212. Marine. 

 England. 



P. arcuatum (Donkin). — Turgid-lan- 

 ceolate, straight, with long, very slender, 

 strongly arcuate beaks curved in con- 

 trary directions ; striae obscure ; median 

 line"^ straight, central. TM. \i. p. 25, 

 pi. 3. £ 10. Marine. England. Closely 

 allied to P. macrum, but distinguished 

 from it by the long, strongly arcuate 

 beaks. Dry valves very pale-brown. 

 (Donkin.) 



P. distortiim (S.). — Stout, turgid-lan- 

 ceolate, produced into short, broad, ob- 

 tuse, subrostrate extremities, which are 

 abruptly bent in contrary directions ; 

 transverse striae obscure, 75 in -001" ; 

 median line central. SD. i. p. 67, pi. 20. 

 £210. Marine. England. Small; colour 

 pale pink. 1-320". 



P. uEstuarii (Br^b., S.). — Broadly 

 lanceolate, rapidly tapering into sub- 

 rostrate, obtuse ends ; median line di- 

 agonal, submarginal near the ends ; striae 

 oblique, 54 in -001" ; colom* pale purple. 

 SD. i. p. m,^ pi. 31. £ 275. = Navi- 

 cula ^stuarii, KA. p. 890; Gyrosigma 

 jEstuarii, Breb. Marine. Europe. 

 Bather small; 1-290". Habit of P. 

 Thuringicum, but smaller, paler, and 

 without the marginal notch, Klitz. 



P. littorale (S.). — Turgid-lanceolate, 

 rapidly attenuated into the cui-ved, sub- 

 rostrate, somewhat acute ends ; longitu- 

 dinal striae conspicuous, 24 in *001", 

 transverse 50 in "001" ; colour pur- 

 phsh. SD. i. p. 67, pi. 22. £ 214. 

 Marine. Eiu-ope. 1-200" ; median line 

 subcentral. 



2 * Valves gihhous at the middle, 



P. Sineiisis (E.). — Large, elongated, 

 broadly linear, flexuose, sigmoid, with 

 gibbous centre and broadly roimded, 

 somewhat incrassated ends, which are 

 curved in contrary directions. = AV/nV«//a 



