48 



On the Measurement of the Stride of Diatoms. 

 By J. D. Sollitt, Esq., Hull. 



A paper having appeared in the ' American Journal of 

 Science 5 for March, 1859, by W. S. Sullivant and T; G. 

 Wormley, on the " Measurement of the Striae of Diatoms/' 

 I think it necessary to make a few remarks on their obser- 

 vations, in order to endeavour to prevent the spread of 

 far greater errors on this interesting point than those which 

 they consider they have corrected. As the subject now 

 stands, it is very similar to the men with the chameleon ; and 

 had Messrs. Sullivant and Wormley, before publishing their 

 paper, made themselves better acquainted with all the varia- 

 tions in the markings on the Diatomaceae, they would have 

 come to a different conclusion to what they have done. To 

 show that this is the fact, I will first call attention to the 

 Pleurosigma fasciola. Of this diatom the kind we find near 

 Hull are very small, and consequently the markings extremely 

 fine, the finest being, as I have before stated, 90 in the 

 T ^5oth of an inch ; but the Fasciola we get from Boston, in 

 Lincolnshire, are many of them so large that there are not 

 more than 50 striae in the TisSoth of an inch. I have fre- 

 quently seen the longitudinal and cross striae at the same 

 time on the largest-sized Fasciola, from Boston, with the 

 half-inch objective, either of Ross or of Powell and Lealand ; 

 but the finest we find at Hull will defy even Powell's one- 

 sixteenth, with 175° of angle of aperture, to show the same, 

 use what illumination you may. The length of the finest Hull 

 Fasciola is under g^th of an inch, whilst some of the Boston 

 Fasciola measure more than -i£jjth of an inch in length. 

 Again, in the P. striyosuni, I may observe that those from 

 the coast of Sussex have their markings very strong, even 

 stronger than set down by Messrs. Sullivant and Wormley ; 

 but those we find at Hull, and which were first named Strigo- 

 sum by Mr. Harrison, in 1846, have exceedingly fine mark- 

 ings on them, varying from 72 in the j 503th of an inch 

 up to nearly 80. With regard to the striae on the NUzschia 

 siymoidea, we have another very striking instance of how 

 little wc know respecting the markings on the Diatomaceae. 

 I have one slide of this diatom, on some of which the mark- 

 ings are very strong and easily seen, whilst others, in the 

 same slide, set at defiance cverv method of illumination to 



