154- HENDKV, OX THE HULL PLICUROSIGMA TASCIOLA. 



striae are doubtless continued to the very extremity of the 

 filiform processes, seemingly bracing them together. 



On comparison of the above measures, it may be observed 

 that the recent Hull gathering contains both the smallest 

 and the largest recorded varieties of Pleurosigma fasciola ; 

 and I am yet sceptical as to the existence of 90 striae iu 

 •001" as published by Mr. Sollitt, or even as to the possi- 

 bility of counting such high numerical quantities, with any- 

 thing like approaching to accuracy, with the best -r^th. inch 

 objective yet to be found in Hull, 



The recent gathering was obtained by a few sweepings of 

 the spoon, within the limited range of a few inches ; so that 

 the latter circumstance, when taken into consideration 

 with the latitude of development and comparative striation, 

 is a matter of no small importance. Had the two kinds 

 of large and small been obtained at some distance apart, one 

 might have imputed the difference to some diverse develop- 

 mental agency, as the nature of soil — quality of water — greater 

 or less abundance of silica present — influence of light, currents, 

 gaseous emanations, or other modifying influences upon the 

 cell-germs or sporangia, or some chemical, \'ital, or mole- 

 cular forces of whatever kind ; but the questio vexata may yet 

 remain as to these extreme developments being due to 

 different stages of growth of the same species, or to the same 

 species under " peculiar idiosyncracy of the sporangial frus- 

 tule''' (Wallich), or to their belonging to some other variety 

 of Fasciola not yet recorded as possessing any distinguishing 

 features. 



One important distinction between the large and small 

 Fasciola consists not so much numerically in the transverse 

 striation, as in the coarse and widely distant longitudinal 

 striae; the one tabulated above as having 35, and the other 

 66 longitudinal strife, in '001". — A ditf'erence which at once 

 accounts for the ready resolution of the former with com- 

 paratively low powers of objectives, and for the difficulty of 

 effecting the resolution of the finer or latter variety (longi- 

 tudinal and transverse striae simultaneously), even with the 

 highest powers, although, in solitary instances, the ith or 

 the -fVth inch will occasionally resolve the twofold striation 

 of the latter in a faint degree. 



The interlinear spaces or areolae, from the nature of the 

 markings and corresponding measures thus afforded in the 

 large Fasciola, necessarily constitute parallelograms having 

 the longer axis disposed in the transverse direction of the 

 diatom ; whilst in the smaller Fasciola, from the numerical 

 estimate of its longitudinal striae, equalling or otherwise 



