PLEUROSIGMA. 



[ 607 ] 



rLEUlU)8IGMA. 



dinal line, and a nodule in the centre and 

 at each end. 



Theniedi.in line and nodides con-ist of an 

 internal tliiekening of the valves; at the cor- 

 responding parts ; the line is best seen in 

 the front view (PI. 1-j. fiy. 16); it is occa- 

 sionally found in a iractured valve, pro- 

 jecting as a solid highly refractive rod, the 

 thinner adjacent portions of the valve being 

 broken aAs ay ; for brevity, it may be called 

 the keel. 



The valves exhibit spurious strife, arising 

 from the existence of rows of dots, of which 

 we have already treated under Diatoma- 

 CBJE. The.=?e s'trias 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 Axgular aper- 

 ture ; the preliminary preparation of the 

 valves is also essential (Diatomace^, p. 

 2.30). 



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 foinid enveloped in amorphous 

 mucus, and those of one species have been 

 found within gelatinous tubes. 



Many species have been described, of j 

 which those that have been used as Test- 

 objects will be enumerated. The mea- 

 surements are mostly those of W. Smith 

 and E. Beck, with which our own have 

 coincided very nearly. The species are 

 arrarged according to the finene.'-s of the 

 markings, which coincides with the diffi- 

 culty witli which they are detected and 

 resolved into dots ; and the appended figures 

 express the number of strife or rows of dots 

 in 1-1000". 



Stfice oblique (dots alternate or quin- 

 cuucial, PI. 15. fig. 40). 



P. formosmii (PL 15. fig. 2o). Broadly 

 linear, attenuated towards the ends ; sig- 

 moidure evident ; keel oblique ; length 

 1-60" ; stria; 36. INlarine. 



P. decorum (PI. lo. fig. 26). Ehomboid- 

 linear ; attenuated ; sigmoidure very evi- 

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

 Marine. 



P. speciosmn {fig. 28). Linear-lanceolate ; 

 sigmoidure resulting from the curvature of 

 one ma: gin 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 l-!iO"; marine: striaa 44. The 

 halves of the valves somewhat resemble the 

 blade of a pocket-kuif.'. 



P. strigosion (tig. 29). Linear-lanceolate; 

 ends rather obtuse, sigmoidure t-light ; keel 

 nearly straight in the middle, curved near 

 the ends; length 1-00"; stria; 45. Marine. 

 Fig. 40 represents the striie resolved into 

 dots. 



P. quadrat mn (fig. 34). Rhoiuboidal, acu- 

 minate at the ends; sigmoid ui'e evident 

 towards the ends ; keel curved, nearly me- 

 dian ; length 1-150" : marine : strite 45. 



/'. elonyatum (PL 15. fig. 31, and PL 1. 

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

 moidure slight, uniform ; keel median ; length 

 1-80" ; marine ; strife 48. 



P. rigiduin (fig. 30). Linear-larceolate, 

 obtuse at the ends ; sigmoidure slight ; keel 

 nearly median; length 1-70"; marine; 

 stri;T3 48. 



P. anf/ulatum (PL 15. fig. .33). Rhomboid- 

 lanceolate or anguUu-lanceolate ; sigmoidure 

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

 marine; stria; 52. PL 1. fig. 16" represents 

 a valve with the strife resolved into dots ; 

 PL 15. fig. 41 represents the dots very 

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

 bits the appearance of hemispherules, which 

 some authors consider to form the true 

 structure. 



PL 15. fig. 33 a represents a specimen 

 with the endochrome and nucleus. 



/3 (fig. 33 6). Simply and narrowly lan- 

 ceolate, ends acute. 



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

 flexed. 



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

 abruptly tapering, short and beak-like; sig- 

 moidure evident ; keel not median ; length 

 1-250" ; marine ; strife 54. 



P. inter meditim (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; stripe 55. 



/i P. nuhecvla. Ends obtuse ; slightly 

 more lanceolate, and shorter; marine; 

 strife 55. 



P. delicatnlum (fig. 32). Very nairowlv 

 linear-lanceolate ; sigmoidure evident ; keel 

 nearly central; marine; length 1-1.30"; 

 strife 64. 



P. obscurum (fig. 27). Linear, attenu- 

 ated near the ends ; sigmoidure shght ; 



