396 DESCEiPTioN or lPo7i/(/astr{ca. 



eluded to be composed of two lamina, or an outer and inner mem- 

 brane, the distance between the two being its thickness. Ehrenberg 

 inclines to the belief that the striae and pinnules are but furrows 

 upon or within the lorica ; but Schleiden represents them as clefts, 

 penetrating between the outer and inner lamina, or lamella (P. 18, 

 f. 1 to 6.) In this matter, too, Scbleiden is supported by the inde- 

 pendent testimony of the Eev. "W. Smith (Ann. Nat. Hist., vol. 7, 

 1851, p. 8) who describes thecostoe of Surirella, &c., as tubes passing 

 between the siliceous valves and the inner membrane ; but to this, he 

 adds, " that those canals communicate with the exterior by a series 

 of perforations," (p. 401.) 



The clear longitudinal bands or iillets, seen in many JVctvicnIa, are 

 also held by Schleiden to be clefts like the pinnules (P. 18, f. 2, 3, 5) 

 but, by Dujardin, they are considered to be elevations or thickenings. 



The prevailing opinion is, that the frustule or lorica of Kavicul^, is 

 inherently one-celled, without any internal subdivisions. The double 

 contour, which denotes the thickness of the wall of the shield (lorica) 

 may be seen to terminate suddenly both above and below." " This," 

 says Schleiden (Principles of Botany, translated by Dr. Lankester, 

 1849, p. 594), "clearly shows that a passage exists from the top to the 

 bottom of the shield." This structure is still better shown by an 

 oblique section, which may be obtained by taking some of the sili- 

 ceous earth of Erbsdortf, and mixing it with mucilage, and, before 

 it is perfectly hardened, cutting off delicate plates with a razor 

 (P. 18, f. 4.) 



Now it has been proved, that cells, whether animal or vegetable, 

 have, at some period of their existence, within them, mostly seated 

 on one part of their wall, a small circular body, called the nuclens or. 

 cytoblast, and which Schleiden conceives to precede the cell itself, 

 and give origin to it. Among the Algae, till of late, this organ was 

 known only in Spirogyra, biit Nageli (Pay Society, 1845, p. 221) 

 affirms that, " in a species of Navicula, in the centre (whether lying 

 on the membrane or free, I know not), is a nucleus with a nucleolus." 

 (See genera Gallionella, and p. 377.) 



This genus Navicuh, from recent researches, now numbers so many 

 species, that, for convenience of description and reference, it has 

 become necessary to break it up into several sub-genera. Ehren- 



