182 



DIATOM-TESTS. 



the fineness of the markings of specimens of the same species 

 obtained from different localities, — a statement now so abundantly 

 confirmed as to be entitled to rank as an established fact. Of the 

 first ten of the foregoing, good specimens may be resolved, with 

 judicious management, by good l-4th or l-5thinch Objectives, and 



A 



Valve of Surirella gemma, with portion more highly magnified, showing 

 two systems of markings a and b. 



even, with very Oblique illumination, by Objectives of half and 

 4-10ths inch, having an angular aperture of 90° ; the remainder 

 require the l-8th or l-12thinch for the satisfactory exhibition of 

 their markings. Several very difficult tests of this description have 

 been furnished by the late Prof. Bailey* of West Point (U.S.), 



on the striae of other Diatoms." Having himself (through the kind- 

 ness of Mr. Lobb) seen " regular, distinct, and unmistakable striae " in 

 this Diatom, the Author cannot but believe in their existence, not- 

 withstanding the failure of the American observers. He has not been 

 able to satisfy himself, however, of the correctness of Mr. Sollitt's; 

 estimate of the distance of the stria;, and is still disposed to regard it 

 as too high. 



* See his interesting Memoirs in Vols. ii. and vii. of the " Smithsonian. 

 Contributions to Knowledge." On Hyalodiscus siibtilis, see Hendry, in* 

 "Quart. Joum. of Microsc. Science," Vol. i. N.S. (1861), p. 179. 



