234< HENDRY_, ON NAVICULA RHOMBOIDES. 



Upon Slide III the same remark holds goodj for No. 6 

 diatom, being the greatest in size, measuring Trhrrd of an 

 inch in length, and bearing 47 striae in "001 ; upon the same 

 slide. No. 1 diatom, being the smallest, and the length of 

 which is only ^-y^th of an inch, exhibits only 50 striae in •001. 



The two preceding specified slides are undoubted Ameri- 

 can, and if we refer to No. 4 slide, a truly English specimen, 

 all ambiguity upon this matter is set at rest by the gather- 

 ings or deposits so widely apart yielding to the same con- 

 clusion; for while the smallest frustules thereupon, being 

 Nos. 2, 3, and 9, yield respectively lengths of xo-otli^ -s-rroth, 

 and xtu^li of ^^ inch, their striae are, 46, 46, and 50 in '001, 

 whereas the largest diatoms, being Nos. 6 and 7, and of 

 lengths jJ-y th and -rJ-^jth, exhibit striai of only 43 in '001 ; and 

 not having witnessed the more subtile markings upon the 

 smaller species, whether English or American, it may be 

 chiefly upon the larger developments of either that a finer 

 striation may be carefully sought for. 



In seeking for striae in this class of objects it is abso- 

 lutely essential that the slide should be well filled, in order 

 to obtain every advantage of position, &c., the diatoms 

 thoroughly cleaned, uniformly distributed, and mounted free 

 of all vapour and moisture. I owe much in these respects to 

 the examples furnished me, from time to time, tlirough the 

 kind generosity of Geo. Norman, Esq., of Hull, whose gene- 

 ral attention and mastery over these details is without a 

 parallel. 



And now, having turned our attention to several diatoms, 

 as Amphipleura pellucida, Navicula rhomboides, and others 

 thus placed at the extremity of our ordinary list of test- 

 objects because of their supposed numerical value and uni- 

 form character of striation, and believing it to be impossible 

 to evade or to gainsay the conclusions at which I have arrived, 

 the question yet remains, — Whence shall we in future derive 

 our standard test-objects? 



Shall we cling to diatoms still, if, peradventure, we may 

 find a species in form, development, and striation, more con- 

 stant ; or hence, quitting nature's treasury, shall we fall back 

 upon the resomxes of human skill ? 



