(Revieiv of 'Recent Geological Literature. 55 
Reviewing Nathorst's experiments with Avorms, pebbleS; twigs, etc., by 
which he reproduced so many problematic genera, he shows conclusively 
that these artificial algse are always found on the lower surface of the 
strata, never penetrating into the substance of the rock, but always ap- 
pearing in "demi-relief." He observes that they possess the precise 
characters of the first category, a fact of much importance. With regard 
then to so many as fall into this list he agrees with the opinions of Dr. 
Nathorst, unless it may be concerning Cruziana, whose form is unlike 
the traces of any known animal. 
By fossilization in " demi-relief " Mr. Maillard means the natural mould 
of an impression. It is a bas-relief and like the latter it can only be 
superficial. After discussing the observations and evidences brought 
forward by Saparta to show that these demi-reliefs are fossil plants, he 
insists that if these plants could fossilize in demi-relief, it ought to con- 
stitute wherever it occurs a generality ; and, secondly, that these reliefs 
shovild occur on the upper face of the strata since the contrary is at var- 
iance with the laws of fossilization. On the other hand their presence 
on the inferior surface proves that, under the probable conditions of de- 
position, there must have been creases or moulds in the matrix which 
were filled by sediment, and which were made by a mechanical agent 
whose nature may or may not have been organic. Accoidingly he ex- 
cludes all those forms in "demi-relief," which constitute the first cate- 
gory, from all possibility of vegetable origin. Passing to the second 
category the author calls attention to the fact that none of the genera in 
the first ever contain characteristics of the other, and that, while the 
representatives of the first do not show the least evidence of a chemical 
or mechanical difference in structure from the matrix, in the other by 
careful experiment he has never failed to show a difference of composi- 
tion. To this he has brought further evidence by the use of the micro- 
scope, showing in many cases the arrangement of the cells as well as a 
typical dichotomy. 
It is impossible to give here an outline of the author's discussion of the 
occurrence and habitat of fossil Algjc, or the conclusions as to the nature 
and affinities of those problematical species to which he devotes special 
consideration. Enough has been said to form a suggestive hypothesis 
for observation. As to the former, he favors the transportation theory ; 
in the latter he argues that all attempts at a systematic classification are at 
present futile since not more in fossil than in living alga; can species or 
genera be based merely on form ; but that larger series must be used, 
combining also the study of the cells, the fruits, and the correlation of 
the organs. Instead of speaking of genera or species of fossil algtc, he 
urges that, except in perhaps the Diatomaceie, and those calcareous algic 
in which the structure is preserved, the use of the word form be adopted. 
In concluding the memoir he gives five well executed plates illustrating 
his categorical classification by artificial "demi-reliefs," formed by the 
method employed by Nathorst, as well as by traces of tertiary animals 
and true fossil algx". 
