120 Prof. J. W. Spencer—Ice Action in the North. 
Ludwigia Murchisone. 
bp 55 var. obtusa. 
cornu. 
or, 
Lioceras elegans. 
99 opalinum. 
% 99 var. comptum. 
If he wrote Ammonites Murchisone, Am. Murchisone obtusus, and 
Am. Murchisone cornu, he would place Am. cornu in the same rank 
as the variety of Murchisone, which would not meet the facts of the 
case. If he leaves it Am. cornu, he does not show its relationship to 
Am. Murchisone, as I do by employing the word Ludwigia. ‘Then 
would he write Am. elegans, Am. elegans opalinus, and Ammonites 
elegans opalinus comptus? If so, I fancy the system would be a more 
cumbrous obstacle to a beginner than the employment of many 
generic names; but if he wrote Am. elegans comptus, he would not 
state the facts of the case, which are, that comptus is a variety of 
opalinus, not of elegans. Then, what rule would be followed with 
regard to choosing the Ammonite which should be the leading one of 
the group ? Surely that which selected the oldest known member, the 
one from which probably the others had descended, because an older 
species could hardly be placed subordinate to a younger one, which 
would happen by any other method. Therefore we choose elegans to 
be the dominant name, instead of using the new generic name Lioceras; 
but supposing we find a Ivoceras older than Am. elegans, and such a 
thing is not unlikely to happen, then we must again change the 
dominant name throughout the group. 
As regards the obstacles to beginners that these names present, I 
would remark that to them these changes do not present the same 
difficulty as they do to one who has been accustomed to an older 
system, and then has to unlearn and learn again. ‘The beginner, 
never having learnt an old system, does not find the new one more 
unfamiliar than the old. Besides which, the use of these generic 
names is intended to point out and emphasize the differences in 
Ammonites which are now passed over owing to their being grouped 
in such large numbers. 
Finally, with regard to the zones ; we have not reached perfection, 
and must effect changes of name as our knowledge increases. I, too, 
can point out a zone that is in danger, viz. the well-known Sauzet- 
zone, because it seems most probable that the Am. Sauzei, d’Orbigny, 
was previously figured as Am. contractus, Sowerby ; but d’Orbigny’s 
figure being so much more easily recognized, his name came into 
more general use. 
VIII.—Nores vron Icz Action 1x Hicn Latirupss. 
By Professor J. W. Spencer, M.A., Ph.D., F.G.S. 
ae notes are intended as an appendix to “‘ Notes on the Erosive 
Power of Glaciers, as seen in Norway” (vide this MaGazinuy, 
Decade III. 1887, Vol. IV. No. 4, p. 167). 
1. The object of my visit to Norway and the Alps was to leave 
