294 PROGRESS OF MICROSCOPICAL SCIENCE. 
was a difference in the power of resisting high temperatures exhibited 
between the mature and young forms of monads. This fact is of in- 
finite importance, as it tends to overthrow Bastian’s notions. 
The Diatom Earth of Richmond, U.S.A.—This subject has been 
discussed in an opening article (by Mr. C. L. Peticolas) in a late 
number of the ‘ American Journal of Microscopy.’ It says that to the 
microscopist this deposit is a source of unfailing interest, whilst the 
most inexperienced in such matters, upon being shown the wonderful 
forms found in it, are struck with surprise and delight. In looking 
at these different forms, one is struck with the wonderful resemblance 
which they bear to things of every-day use, as among them may 
be found models of almost all the implements used by savages, 
whether for war, the chase, or in domestic life; witness, for instance, 
his stone hatchets, arrow and spear heads, knotted clubs, boomerangs, 
&c.; a catalogue of such matters used by civilized people would 
embrace plates, dishes, cups, saucers, gridirons, pins, balls, tops, 
spectacles, watches, anchors, dumb-bells, cannon, coin, musical notes, 
and many other articles—the investigator being constantly startled 
by the strange resemblance which hundreds of these ancient natural 
forms bear to articles used in our houses and workshops. Certain 
varieties, however, predominate, and their distribution varies with 
level and locality, the upper portion of the stratum being compa- 
ratively poor in forms, while they increase in number and variety as 
we descend to the middle, falling off again towards the bottom. The 
genus Coscinodiscus seems to characterize this earth, and of it there 
are dozens of varieties varying from the (microscopically) enormous 
C. gigas to the minute and elegant C. subtilis, which resembles closely 
a finely polished opal, requiring a lens of wide aperture and consider- 
able power to show its revelations. Orthosira marina is abundant, 
whilst many beautiful forms of Navicula are found in every gathering. 
Amongst these we may note two kinds of Pleurosigma, one of which, 
P. angulatum, is a favourite test-diatom, and the other, which it is pro- 
posed to call P. Virginica (as it is the most common form of Plewrosigma 
in the Virginia earths), is remarkable for the beauty of its contour, 
which exactly copies a willow leaf, and the want of uniformity in its 
strie, which are much coarser in the middle than at the ends of the 
valves. It is easily resolved with a }-inch objective, without the aid 
of oblique light. The genus Triceratium is also well represented by 
many beautiful varieties, the most interesting of which is T. obtusum, 
which can be resolved as easily as P. Virginica. Isthmia enervis, Bid- 
dulphia Turmegti, Terpsinoe, Musica, Aulacodiscus crux, Navicula lyra, 
Gonphonema, Heliopelta, Asterolampra concinna, Asteromphalus Brooketi 
and Synedra are comparatively rare. From the great variety of its 
species, and the wide range in the character of their markings, the 
Richmond earth forms one of the best and most interesting tests 
for the performance of objectives of almost every power. On some, 
for instance, the areolations may be seen with a simple triplet, magni- 
fying 25 linear; on others a first-class twelfth or sixteenth of wide 
angular aperture, aided by all the modern refinements of illumination, 
is needed to show them. 
