Navicidacea.] infusoeial animalcules. 399 



Kiitzing, uiul adopt the term Pleurosigma, as descriptive of another 

 group. The genus Navicida of Kiitz., and other writers, will then 

 be resolved into three, whose characters may be given as follows : — 

 1. I'leurosigma. Valves convex, sigmoid striated; striae resolvable into 

 dots. (P. 20, f. 17, 18, 19.) 2. Navicula. Valves convex, lanceolate 

 or elliptical, smooth or striated ; striae resolvable into dots. 3. Pinnu- 

 laria. Vahes convex, oblong or elliptical, ribbed or pinnated with 

 distinct costae, not resolvable into dots. ..." In Pleurosigma, the 

 resolution of the striae into their constituent beads becomes a task of 

 extreme difficulty, and has, from this circumstance, been very 

 generally adopted by microscopists as a means of testing the object- 

 glass of a microscope. The presence of striae, on the valves of 

 Pleurosigma and Xaricula, may be known, even when the power em- 

 ployed is insufficient to detect lines, by the colour of the dessicatedfrus- 

 tulcs viewed by transmitted light. This colour diifors in each species ; 

 it arises from the refraction of the rays j^assing through the siliceous 

 plate, and its shades depend on the direction of the striae, and their 

 distance from each other ; its aid may therefore be evoked in the 

 discrimination of species, and will sometimes be found the most 

 facile and certain means of identification. 



" In the Xacicuhce generally, each valve is traversed by a median 

 line, across which the striae do not pass. The centre and extremities 

 of the line are somewhat enlarged, and these enlargements have 

 been regarded by many wi'iters as openings in the siliceous plates. 

 I have never been able to satisfy myself that such openings exist, 

 and am disposed to regard the line itself, and its enlargements, as 

 peculiarities Kttle connected with the essential structure or functions 

 of the cell. More important, in a structural point of view, is the 

 form of the connecting membrane, which, in Pleurosigma, consists of a 

 narrow ring of silex, and which in no period of its growth appears to 

 have any very considerable development. The consequence is, that 

 the frontal (lateral, Ehr.) view of the frustules is uniformly of a 

 linear, or, when the convexity of the valves is considerable, of a 

 linear lanceolate form, while in Navicula and Pinnularia (P. 1 8, f. 2, 

 20, 22, 23), as the connecting membrane is often more fully de- 

 veloped, the front -vdew of their frustules is frequently oblong or 

 quadi'ilateral. Two much importance must not, however, be attri- 



