PLEUBOSIGMA. 



[ 607 ] 



PLEUROSIGMA. 



dinal line, and a nodule in the centre and 

 at each end. 



The median line and nodules consist of an 

 internal thickening of the valves at the cor- 

 responding parts ; the line is best seen in 

 the front view (PI. 15. fig. 16) ; it is occa- 

 sionally found in a fractured valve, pro- 

 jecting as a solid highly refractive rod, the 

 thinner adjacent portions of the valve being 

 broken away ; for brevity, it may be called 

 the keel. 



The valves exhibit spurious strise, arising 

 from the existence of rows of dots, of which 

 we have already treated under DIATOMA- 

 CEJE. These strise and dots are in most 

 species very difficult to detect, requiring 

 the use of "oblique light, and the modern 

 condensers with the stops ; the principles 

 which should guide in the search for them 

 have been explained under ANGULAR APER- 

 TURE ; the preliminary preparation of the 

 valves is also essential (DIATOMACE^;, p. 

 250). 



Most of the species are found in salt or 

 brackish water ; a few in fresh water. They 

 often abound upon the surface of mud. 

 Conjugation or the formation of sporangia 

 has not been observed. The frustules are 

 sometimes found enveloped in amorphous 

 mucus, and those of one species have been 

 found within gelatinous tubes. 



Many species have been described, of 

 which those that have been used as TEST- 

 OBJECTS will be enumerated. The mea- 

 surements are mostly those of W. Smith 

 and R. Beck, with which our own have 

 coincided very nearly. The species are 

 arrarged according to the fineness of the 

 markings, which coincides with the diffi- 

 culty with which they are detected and 

 resolved into dots ; and the appended figures 

 express the number of strife or rows of dots 

 in 1-1000". 



Stria oblique (dots alternate or quin- 

 cuncial, PL 15. fig. 40). 



P. formoswn (PI. 15. fig. 25). Broadly 

 linear, attenuated towards the ends; sig- 

 moidure evident ; keel oblique ; length 

 1-60" ; strise 36. Marine. 



P. decorum (PI. 15. fig. 26). Rhomboid- 

 linear ; attenuated ; sigmoidure very evi- 

 dent; keel oblique; length 1-90"; strise 36. 

 Marine. 



P. speciosum (fig. 28). Linear-lanceolate ; 

 sigmoidure resulting from the curvature of 

 one maigin of each half of the valve, the 

 opposite margin of each respective half 



being nearly straight; keel in each half 

 forming two curves ; very oblique near the 

 ends ; length 1-90" ; marine : striae 44. The 

 halves of the valves somewhat resemble the 

 blade of a pocket-knife. 



P. strigosum (fig. 29). Linear-lanceolate ; 

 ends rather obtuse, sigmoidure slight ; keel 

 nearly straight in the middle, curved near 

 the ends; length 1-90"; striae 45. Marine. 

 Fig. 40 represents the strise resolved into 

 dots. 



P. quadratum (fig. 34). Rhomboidal, acu- 

 minate at the ends; sigmoidure evident 

 towards the ends ; keel curved, nearly me- 

 dian ; length 1-150" ; marine ; striae 45. 



P. elongatum (PI. 15. fig. 31, and PI. 1. 

 fig. 18). Linear-lanceolate, acuminate ; sig- 

 moidure slight, uniform ; keel median ; length 

 1-80" ; marine ; strise 48. 



P. riyidum (fig. 30). Linear-lanceolate, 

 obtuse at the ends ; sigmoidure slight ; keel 

 nearly median; length 1-70"; marine; 

 strise 48. 



P. angulatum (PI. 15. fig. 33). Rhomboid- 

 lanceolate or angular-lanceolate ; sigmoidure 

 evident; keel nearly median; length 1-110"; 

 marine; striae 52. PI. 1. fig. 16 represents 

 a valve with the striae resolved into dots ; 

 PI. 15. fig. 41 represents the dots very 

 highly magnified ; and PI. 15. fig. 46 exhi- 

 bits the appearance of hemispherules, which 

 some authors consider to form the tnie 

 structure. 



PI. 15. fig. 33 a represents a specimen 

 with the endochrome and nucleus. 



/3 (fig. 336). Simply and narrowly lan- 

 ceolate, ends acute. 



7 (fig. 33 c). Ends beaked; abruptly 

 flexed. 



P. cestuarii (fig. 35). Lanceolate ; ends 

 abruptly tapering, short and beak-like ; sig- 

 moidure evident ; keel not median ; length 

 1-250" ; marine ; striae 54. 



P. intermedium (fig. 36). Narrowly 

 linear-lanceolate, acute ; sigmoidure none, 

 ! or merely indicated by a slight inequality 

 in the opposite margins of the valves : keel 

 nearly straight and almost median ; length 

 1-140"; marine; striae 55. 



ft P. nubecula. Ends obtuse; slightly 

 more lanceolate, and shorter ; marine ; 

 striae 55. 



P. delicatulum (fig. 32). Very narrowly 

 linear-lanceolate ; sigmoidure evident ; keel 

 nearly central; marine; length 1-130": 

 stria/ 64. 



P. obscm-um (fig. 27). Linear, attenu- 

 ated near the ends ; sigmoidure slight ; 



