94 E. M, NELSON ON NAVICULA RHOMBOIDES AND ALLIED FORMS. 



and we know that the mahogany slip was used by Topping 

 in 1841. In Messrs. Sollitt and Harrison's celebrated paper 

 (read before the British Association at Hull in 1854) there 

 is no mention of either the Amician Test or of J\^ rhomboides. 

 Their work being principally connected with marine and brackish 

 forms may account for the omission of N. rhomboides from their 

 list. 



Gregory's list of 1853 contains N. o'homhoides, and that of 1855 

 both rJtomhoides and crassinervis. Koper's list of 1854 contains 

 crassinerins and cusjyidata, but not rhomboides. In 1868 

 M. Mouchet writes* that ''' Navicida affinis and N. rhomboides 

 do not resemble each other in any way, either in form or in 

 the fineness of their striae. Navicula affinis is always dis- 

 tinguished by the line or nervure running along the margins 

 of the valve, which is gently contracted towards its extremities 

 and the ends of which are rounded oft*. The striae, although 

 difficult to resolve, are much less closely packed (46-60 in 0001'') 

 than those of N. rhomboides. Different authors, however, have 

 described and drawn the one for the other. The opticians often 

 give to N. affinis the name of N. amici, no doubt because this 

 diatom was the favourite test of that able microscopist. 

 N, affiiiis is also confounded with the A", gracilis, N. rhombica, 

 JV. cuspidata, etc., in such a way that it is sometimes difiicult 

 to recognise them. I have said that the two diatoms in question 

 ought not to be confounded. In fact, whilst the A. affinis, with 

 the elliptic valve, is pinched up towards its ends, it is quite 

 otherwise with A. rhomboides, which has a nearly quadrangular 

 form, and the ends of which are lanceolate. The striae of this 

 diatom (85 in O'OOl") make it a test of the first order." Enough 

 has been said to show that it was after the Exhibition of 1851 that 

 A", o'homboides, crassinervis, etc., began to be talked about, and 

 diatoms used as test-objects ; so these two particular slides may 

 very well be assigned to a date about 1855. We are, however, 

 at jjresent more concerned with what is on the slide, than about 

 their date. These slides have gone bad, i.e. sweated, etc., and would 

 not have been preserved had it not been for their historic interest. 



A rough examination shows a number of A. rhomboides^ a few 

 large ones, a few small ones, and many of a medium size, so that 

 if you want a very large or a very small one you have to hunt 

 * Quart. Journ. Mic. Sci., ser. 2, vol. 8, p. 105 (1.SG8). 



