116 SULLIVANT AND WORMLEY^ OX NOBERt's TEST-PLATE. 



■; Xiimber of Btri* in (tOl'. 



ir. and S. Sni. Syn. S. and "W. 



Navicula rhomboides ... 60 to 1 1 1 85 70 



Pleurosiyma fasciola . . . 50 to 90 61 5.2 to 56 



Pleurosigma strigosum . . 40 to 80 14 13 



Niizschia sigmoidca ... 105 85 70 



^lany frustules of these species, from different localities, 

 liave lieen measured by us, and always with the same results. 

 Pleurosigma fascioJa has been specially designated by INIi*. 

 Sollitt, and also by Dr. Wallich, as very inconstant in its 

 markings. Of this diatom we are fortunate in Ijeing supplied 

 with abundant specimens, from various localities in England, 

 particularly from the neighljourhood of Hull. Several 

 hundred valves, not a few under -,-iTith of an inch in length, 

 were measured, and on no one w^ere found strise less than 52 

 or more than 56 in 'OOl", much the larger number being Si. 

 A similar uniform striation has alwavs been observed among 

 the individuals of many other species examined by us.* 



To such uniformity of striation Amphipleura peUucida 

 forms as yet no exception ; this diatom is still a '^res A'exata" 

 among microscopists ; neither the striation nor the structure 

 of its frustule is at all satisfactorily understood. The record 

 of its striation is found to be thus: — In 1854 Messrs. Harrison 

 and Sollitt's measurements made its striae 120 to LSO in 

 •001". Prof. Carpenter (1856) first suggests the probability 

 of some error in these measureiuents ; the writers of this 

 paper declared themselves (this Jour., March, 1859) unable 

 to ^'glimpse'' the strire. Mr. Sollitt ('Mic. Jour.,' Oct., 1859) 

 measures them again, and finds them still as low as 120 to 

 130 in '001 ", but gives it as the opinion of Mr. Lobb that 

 " even those figures are too low, and that they ought to be 

 set down 'at 140 in 'OOl".'' In the same number of the 

 ' Microscopic Journal/ Mr. Eylands sees " striae, but much 

 more distant than the 130 in "001" of the Hull microscopist." 



* It is well known that among iudividuals belonging to the same species, 

 and on the same slide, some are much more difficult ol'resolulion than olliers. 

 This is owing to the position of tl)e valves, thickness of covering-glass, deptii 

 of balsam, &c., and not to a supposed difference in the number of their 

 striae, as the micrometer will readily demonstrate. 



Estimates of the number of striaj based on a visual comparison with the 

 known striation of other species are seldom reliable : instances of the vague- 

 ness of this method are seen in the valuable paper of Dr. Donkin on 

 Northumbrian Diatomaceae (' Mic. Journal,' 1S5S), where adopting Pleiiro- 

 sit/ma angulutum as a standard, he estimates the strife of liis Pleurosigma 

 lanceolulum at about 70, and those of his Toxomdea insignis at about 75 to 

 80 in •001", whereas in both eases actual measurements show the striae 

 (transverse and diagonal) to be only 57 iu the same space. 



