MAX SCHULTZE, ON THE DIATOM-VALVE. 129 
pression of ‘ raised ridges,” three sets of which, according to 
him, of equal breadth and decussating with each other at an 
angle of 60°, exist in P. angulatum. From this it would 
seem that he had given up the notion that the sets of lines 
were situated at different levels, as “raised ridges” can 
hardly be imagined to exist except on the surface. 
When any one of the three species of Plewrosigma above 
named is examined under a high magnifying power and with 
a central illumination, rows of points will be seen on its 
surface, varying in brilliancy according as the body of the 
instrument is raised or lowered. In P. hippocampus and 
balticum these points are placed in two series at right angles 
to each other, whilst in P. angulatum they form three series, de- 
cussating, as above said, at an angle of 60°. They may be viewed 
either as bright spots upon a dark ground or as dark ones 
upon a bright ground,* and it has been disputed which ap- 
pearance indicates the ‘‘ proper focal distance.” 
Welcker’st excellent, though strangely neglected observa- 
tions, have shown the futility of such a dispute. By them 
we are at once enabled, in the most precise way, to answer 
the next interesting question, viz., as to whether the points 
on the surface are the expression of elevations or of depres- 
sions. The readers of the ‘ Microscopical Journal’ are 
aware that, with respect to this point, much contention has 
been carried on in England, and that the opinions of micro- 
scopists with respect to it have been directly opposite. Whilst 
Carpenter, confessedly one of the greatest English authorities 
on any questions of microscopy, and, with him, many other 
observers, holds these points to be depressions, relying, in sup- 
port of this view, as does Harting, particularly upon the analogy 
with the more coarsely marked diatoms, in which the marking 
on the surface certainly depends upon series of depressions, the 
opposite view has, nevertheless, continued to gain ground, 
and to be advocated by no less skilful observers. Amongst 
the more eminent of these is Dr. Wallich,{ who, by the use 
of oblique light—which he prefers in all cases—believes that 
he has ascertained beyond question, that the marking on P. 
angulatum, balticum, &c., is produced by pyramidal sharp 
facets, and finely acuminate elevations on the surface. 
* Hall, ‘Quart. J. Mic. Se.,’ IV, Pl. XIII, fig. 2. 
Ibid., VII, p. 240, and VIII, p. 52. 
‘Ann Nat. Hist.,’ Feb., 1860. Vide also ‘Q. J. Mic. Sc.,’ VI, 1858, 
p. 247, where it will be seen that in some caes, as, for instance, in 7ricera- 
tium favus, Dr. Wallich recognises the existence of distinct hexagonal cells 
on the surface. Though he is inclined (ib. viii, p. 142) to deny that any 
analogy exists between the marking on that genus and that of Pleurosigma, 
&e. : 
