ISO 
The Afttericau Naturalist. [March, 
branchii and maintained as a distinct class like the Dipnoi. He 
employs Liitken's name, Xenacthini for it, but this must be 
clearly retained for the subdivision of the Ichthyotomi to 
which Xenacanthus properly belongs. If for instance, it should 
be discovered that Acanthodes belongs to the Ichthyotomi, 
(American Naturalist, 1887, p. 1016) the Xenacanthini 
and Acanthodini would be two of its primary divisions. 
It is to be regretted that M. Brongniart was not better ac- 
quainted with the work done in America on this group, as he 
would have been thus spared the necessity of making some 
new names.— E. D. COPE. 
Croll on Misconceptions Regarding the Evidence 
OF former Glacial Periods. In a paper read before the 
Geological Society of London, January 23, 1889, Dr. James 
Croll made the following statement : — 
The imperfection of the geological record is greater than is 
usually believed. Not only are the records of ancient glacial 
conditions imperfect, but this follows from the principles of 
geology. The evidence of glaciation is to be found chiefly on 
land- surf aces, SiVid the ancient land-surfaces have not, as a rule, 
been preserved. Practically, the several formations consist of 
old sea-bottoms, formed out of material derived from the de- 
gradation of old land-surfaces. The exceptions are trifling, 
such as the underlayer of coal-seams and dirt-beds, like those 
of Portland. The transformation of an old land-surface into a 
sea-bottom will probably obliterate every trace of glaciation ; 
even the stones would be deprived of their ice markings; the 
preservation of boulder-clay, as such, would be exceptional. 
The absence of large, erratic blocks, in the stratified beds, may 
indicate a period of extreme glaciation, or one absolutely free 
from ice. The more complete the glaciation the less probabil- 
ity of the ice-sheet containing any blocks, since the rocks 
would be covered up. Because there are no large boulders in 
the strata of Greenland or Spitzbergen, Nordenskiold maintains 
that there were no glacial conditions there down to the termi- 
nation of the Miocene period. The author maintained that 
glaciation is the normal condition of polar regions, and if these 
at any time were free from ice, it could only arise from excep- 
tional circumstances, such as a peculiar distribution of land and 
water. It was extremely improbable that such a state of things 
could have prevailed during the whole of the long period from 
the Silurian to the close of the Tertiary. 
A million years hence, it would be difficult to find any trace 
