ORIGINAL COMMUNICATIONS. 



On Navicula rhomboides. 

 By W. Hendry, Esq., M.R.C.S., Hull. 



Few diatoms are more generally distributed amongst mi- 

 croscopists than Navicula rhomboides, for, being found in abun- 

 dance in a fossil condition, and holding a place high in the 

 rank of numerical striation, it has evidently been eagerly 

 sought after, and as readily supplied, as though its resolution 

 bore the strongest existing evidences of the power and quality 

 of lenses. However, there exists no difficulty in showing 

 that N. rhomboides is the most variable in its dimensions, 

 irregular in its form, indistinct in development, and possesses 

 a striation more extended in its range than any other known 

 and measured diatom, thus totally unfitting it to take rank, 

 under any circumstances, as a test-object. As to its figure, 

 it is as oftentimes arched or lanceolate as it is angular in its 

 outline, and in either case presents the same characteristic 

 median lines and nodules, with transverse distribution of 

 striae. 



Median line straight, with dark ground and double-light, 

 coloured inner bands, forming an X-like junction at the cen- 

 tral nodule when approaching focus, the angles of which 

 being thus continuous with the bands, the latter terminate 

 distinct and within the apices of the diatom in light-coloured 

 small, conical nodules, having their bases central, a third 

 luminous, middle band likewise in some instances appearing. 



If we attempt comparison with other diatoms, it would 

 tend but to confusion, as with N. gracilis, N. rhom- 

 bica, N. ciispidata, &c. I hold some London slides of N. 

 rhomboides synonymised American test, N. gracilis, so 

 that it is somewhat difficult at times to give a correct in- 

 terpretation to language. 



Pacts are stubborn things, and as our subject will not bear 

 too much of the speculative or conjectural, I herewith sub- 

 join a series of measures of these diatoms, as they are pre- 



VOL. I. NEW SER. R 



