Correspondence—Dr. A. Irving. 427 
skiella, and Fistulipora. Of these, Geinitzella, Hexagonella, and 
Dybowskiella are proposed as new, but their characters are so similar 
to those of Stenopora, Evactinopora, and Fistulipora respectively, 
that the author might well have spared the introduction of the new 
names. A very important feature in the description of these Corals 
is the way in which their minute structures have been investigated 
by means of microscopic sections, of which several hundreds were 
prepared by Mr. Wentzel, the colleague of Prof. Waagen in the 
authorship of this memoir. A comparison of the beautiful figures 
given of these sections, with those of nearly allied forms which have 
appeared in the papers of Prof. Nicholson and Mr. Foord, fully 
shows the value and absolute necessity of basing the determination 
of these and other Corals on their minute structural characters. In 
the interpretation of some of these minute structures, the authors 
of this memoir differ considerably from Prof. Nicholson; but we are 
inclined to think that, as regards the nature of the wall in the axial 
corallites, the view of Prof. Nicholson, that it is really double, better 
accords with the facts, than the explanation that it is single, and 
that fracture really takes place between it and the subsequently 
deposited layers of stereoplasm. Further, the evidence seems 
insufficient to establish the statement that the spinous structures 
in many of the Monticuliporide, the Acanthopores of Nicholson, 
are merely the young stages of the ordinary corallites. Other points, 
on which somewhat dogmatic opinions are given, are likewise open 
to criticism ; but we must content ourselves with an expression of 
satisfaction that these organisms have been so thoroughly and 
carefully investigated and described; so that this memoir is a 
refreshing contrast to some of the earlier publications of the Indian 
Survey, in which the superficial features of the Corals merely have 
been noticed. A tabular statement of the species described in the 
memoir would have increased its value and convenience for reference. 
Cade Jal; 
CORRESPONDENCE. 
————— 
THE ATMOSPHERE OF THE CARBONIFEROUS PERIOD. 
Srr,—May I be permitted one word on a question which has been 
raised as to the greater prevalence of carbonic acid in the atmosphere 
of this earth in the Carboniferous Period than at later periods, if 
only to suggest that Prof. Prestwich and his critic seem to be arguing 
at cross-purposes? There is no reason why both statements should 
not be true. The real question would then be, as to what would 
constitute “an excess of carbonic acid.” There is some confusion 
of thought as regards such two essentially different physiological 
functions of plant-life and growth as respiration and assimilation of 
carbon. This is hardly excusable when we need go no farther than 
the most trustworthy elementary books (such as those in the London 
series), to be informed of the essential difference of these two 
processes, and of necessity of free oxygen for the activity of proto- 
plasm in the plant and animal alike. On general grounds therefore 
