Messrs. SuUivant atm? Wormley on Nobert's Test Plate. 21 1 



Many frustules of these species^ from different localities^ have 

 been measured by us, and always with the same results. Pleu- 

 rosigma fasciola has been specially designated by Mr. Sollitt, 

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

 Of this Diatom we are fortunate in being supplied with abun- 

 dant specimens from various localities in England, particularly 

 from the neighbourhood of Hull. Several hundred valves, not 

 a few under ^^Qih. of an inch in length, were measured, and on 

 no one were found striae less than 53 or more than 56 in '001", 

 much the larger number being 54. A similarly uniform striation 

 has always been observed among the individuals of many other 

 species examined by us. 



To such uniformity of striation Amphipleura pellucida forms, 

 as yet, no exception ; this Diatom is still a res vexata among 

 microscopists ; neither the striation nor the structure of its frus- 

 tule is at all satisfactorily understood. The record of its striation 

 is found to be thus : — In 1854, IMessrs. Harrison and Sollitt's 

 measurements made its striae 120 to 130 in -001"; Prof. Car- 

 penter (1856) first suggests the probability of some error in these 

 measurements ; the writers of this paper declared themselves * 

 unable to " glimpse " the striae ; Mr. Sollitt f measures them 

 again, and finds them still as low as 120 to 130 in "OOl", 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 '001";" 

 in the same number of the ' Microscopic Journal ' Mr. Ryland sees 

 " striae, but much more distant than the 130 in -001" of the Hull 

 microscopists ;" lastly, Mr. Hendry | states that he has '^ come 

 to a satisfactory conclusion that it is a sad misrepresentation to 

 set down the lines so high in the scale as 130 in '001", and that 

 on a few shells lines may be counted at 42, and many at 60, 70, 

 and 80 in '001".^' A perplexing record, truly ! reminding one 

 of the celebrated Torbane Hill coal case§. 



It is our impression, notwithstanding these conflicting state- 

 ments, that the Diatom before us presented to all these gentle- 

 men the same appearances, but their interpretations of these 

 appearances have been widely diff'erent. 



The testimony of our objectives, as we understand it, seems 

 to indicate that this Diatom has a minutely and irregularly 

 broken-up surface, which, even on the same valve, can be made 

 to show an apparent striation, varying from moderately coarse 

 to extremely fine, according to the obliquity or intensity of the 

 illumination, and to the grade, whether low or high, of the ob- 

 jective used, thus proving beyond question that the exhibition 



* Silliman's Araer. Journ. March 1859. f Micr. Journ. Oct. 1859. 



• X Micr. Journ. July 1860. § Micr. Journ. ii. p. 64. 



