356 Dr. G. C. Wallich on the Structure of 



not through them, as would naturally occur if they were depres- 

 sions." 



Having thus endeavoured to show that Professor Schultzfi 

 was correct in asserting that great difference of opinion existed 

 in this country on the subject under discussion, it is necessary 

 to cite the evidence on which he gives his adhesion to the belief 

 still entertained, as he erroneously supposes, by Dr. Carpenter, 

 that the so-called " dots " are depressed spaces on the valve of 

 Pleurosigma. 



And first with regard to the electrotype metallic casts of 

 Pleurosigma, of which a notice is contained in the Quarterly 

 Journal of Microscopical Science, vol. iii. p. 244 et seq. In a 

 note appended to Schultze^s paper (Quart. Journ. Mic. Sc. 

 for April 1863, p. 128), the editors state that Mr. Wenham^s 

 experiments do not appear to have been limited to the two 

 species named, as he says that he had " obtained distinct im- 

 pressions of the markings of some of the more difficult Diato- 

 macese, such as A^. {Pleurosigma ?) balticum, P. hippocampus, &c., 

 leaving, as he says, no doubt of their prominent nature. But 

 whether this prominence belongs to the areolce or intermediate 

 lines does not appear." Schultze, in the paper now under 

 notice, does not hesitate to characterize these experiments as 

 perfectly successful, and as having demonstrated that the im- 

 pressions represented '^the systems of lines or dots," thus 

 manifestly supporting the view entertained by Smith in the 

 Synopsis, and most other observers, namely, that the " striae " 

 are resolvable into " dots," an idea I cannot subscribe to, inas- 

 much as it shall presently be my endeavour to prove that the 

 appearance of striation in P. angulatum and the whole of the 

 forms combining diagonal with rectangular striation is engen- 

 dered at the thinnest portions of the valve, through which the 

 rays of light penetrate most readily, and along the lines of which 

 the valve almost invariably fractures ; whereas the so-called dots 

 are produced only when those lines are thrown partially out of 

 focus, the intervening elevated portions of the structure being as 

 partially brought into focus. 



The main source of error and misunderstanding amongst pre- 

 vious writers is traceable to the supposition that the diagonal 

 and rectangular series of lines (as the case may be) constitute 

 the portions of the valves which, under the higher magnifying 

 powers of the microscope, become convertible into the so-termed 

 " dots," " beads," or " hexagons," — the fact being that the 

 " striae " seen under the lower powers, if properly exhibited, 

 are never convertible into anything but hues, whereas the dots, 

 beads, and hexagons are the imperfect expositions of the struc- 

 ture occurring in the spaces included between the intersections 



