42 GENEBAL HISTORY OF THE INFUSOEIA. 



Stauroneisy &c., whilst in Am])M]pleura (XIII. 1, 2) two ridges are noticeable, 

 but whether these are of the same nature structurally is uncertain. In 

 Doryphora, again, there is a median band, but no nodules distinguishable ; and 

 in Eunotia and Himantidium the terminal nodules would seem exceptional in 

 character, being due, as Prof. Smith supposes, *' to an inflection of the valves 

 at the point of junction." The roimded space in the centre of the discoid 

 valves of Actinocyclus (XI. 132) and Arachnoidiscus (XV. 18-21J, which is 

 devoid of areola, is designated by him a pseudo-nodule, in order, we presume, 

 to contrast a mere bare space with the like smooth but condensed and thick- 

 ened spots described as nodules. 



In this record of opinions, those of Siebold and Niigeli (J. M. S. i. 196) 

 should not be omitted : — " Precisely at the spots (says the former writer) at 

 which Ehrenberg and others suppose they have seen six openings (i. e. three 

 on each valve) in Navicula, the silicious cell-membrane is thickened, and con- 

 sequently forms so many rounded eminences which project internally." These 

 views thus far tally with those of Prof. Smith and others ; however, a few 

 lines further on in his essay, Siebold expresses the belief that the lines 

 running along the middle of the sui-faces from one thickening to another 

 " are to be referred to a suture, fissure, or rather gap, in which no silicious 

 matter is deposited, so that in these places the delicate primordial membrane 

 which lines the silicious shield can be brought into close relation ^ith the 

 external world. I come to this conclusion fi'om the circumstance that it is 

 exactly at these four sutures or fissures that the water surrounding the 

 Navicula is set in motion." (See p. 50.) 



Upon the whole question of the actual nature of the markings on the 

 surface of the silicious fnistules, we are happy to add a paper published by 

 Prof. Bailey (SiU. Journ. ii. 349), which appears to afford a satisfactory 

 elucidation. We present it entire, with the practical notes on manipulation, 

 so that our readers may imdertake a critical examination for themselves : — 



" I now offer proof which removes all doubt, and shows that these markings 

 are neither apertures nor depressions, but are in reahty the thickest parts of 

 the shell. If the shells are placed in dilute hydrofluoric acid and watched 

 by the aid of the microscope as they gradually dissolve, the thinnest parts of 

 course dissolve fii'st, and apertures, if any exist, should become enlarged. 

 Now the very parts which have been called orifices by some, and depressions 

 by others, are the last of all to disappear as the shell is dissolved. This mode 

 of observation, besides establishing the fact that these are the thickest parts 

 of the shell, reveal many interesting particulars of structure. Thus, in 

 the large Pinnidarla, it may be seen, with even a low power, that the two 

 parallel bands (separated by a canal) which reach fi'om the central knob to 

 the terminal ones, and which appear smooth before the application of acid, 

 become distinctly striated after their surface is dissolved off, as does also the 

 central spot itself, showing that striae which existed in the young shell are 

 covered up and nearly obhterated^by subsequent deposits. In Staurosira the 

 cross-band and the two longitudinal bands are the last to dissolve, and these 

 last bands, as in most Diatomacea, appear separated by what is either a canal 

 or thin portions of the shell. In Grammatoj>hom the undulating lines are 

 internal plates, which are the last to dissolve. In HeliopeUa, Actinoiytyclms, 

 &c., the polygonal central spot is the last to disappear. In Isthmia, the spots 

 on the surface, which at first appear like granular projections, are in reality 

 thin portions of the shell, and imder the action of the acid they soon become 

 holes. The acid also proves that the larger spots at the transverse bands are 

 a series of large arcuate holes in the silicious shell, and the piers of this 

 series of arches remain some time after the rest of the shell has vanished. 



