544 James Durham—Pleistocene Geology, Firth of Tay. 
though in one instance I find the posterior ridge bifurcating so as to 
form six at the outer margin. The free margins of those ridges are 
prominent and pretty regularly and gently convex, though rather 
more so towards the outer aspect, and are divided into a variable 
number of denticulations, which, when the enamel covering is gone, 
as is often the case, are blunt and rounded ; otherwise these denticles 
are compressed, triangular, trenchant, pointed, and brilliant. The 
furrows between the ridges are smooth. The mandibular tooth 
resembles the palatal one in the number and character of the ridges, 
but is somewhat narrower as well as somewhat more convex in 
general aspect. 
This is a small species, though not so minute as C. angustulus ; the 
largest palatopterygoid with tooth which I have seen measuring 
11 inch from the anterior extremity of the tooth to the posterior- 
external angle of the bone. In shape it most resembles Ci. obliquus 
of Hancock and Atthey, from which it differs, first in its small size, 
secondly in the constant or nearly constant presence of five ridges, 
whereas in Ct. obliquus there are six to eight. Considering, how- 
ever, the great tendency to variation in teeth of the genera Ctenodus 
and Ceratodus, I hardly think it safe to devote quinquecostatus to the 
rank of a species at present.’ 
TV.—Tue Puetstocent Geotocy or THE Firta or Tay AND THE 
« WLEVATION AND SUBSIDENCE ” QUESTION. 
By James Dunnam, F.G.S. 
ITHIN the last few months a good deal has been written upon 
the question of upheavals and depressions in the crust of the 
earth both in the Gro.oeicaL Magazine and in “ Nature.” By far 
the majority of these writers have maintained with some show of 
reason that depressions are the result of the deposition of material 
of some sort on a part of the earth where it was not previously, in 
consequence of which the newly loaded part sinks down, just as a 
weight placed upon an air cushion causes the part on which the 
weight rests to be depressed. This novel view has been supported 
by pointing to the fact that numerous instances are met with in 
which vast thicknesses of rocks have been superimposed one upon 
another, while throughout their whole depth traces of shallow or 
comparatively shallow water are found, it being inferred that the 
sedimentary deposit being the cause of the depression, the bottom 
sinks exactly in proportion to the amount of the deposit. The 
Carboniferous system bas been advanced as a conspicuous instance 
of this state of matters; the same holds true of the lower Old Red 
Sandstone, which appears to have been deposited in lakes. 
It has been further shown that Greenland, which is at present 
covered with ice, is being depressed, and the depression is supposed 
to be directly caused by the great weight of snow and ice which 
buries its mountains and fills its valleys. 
1 For previous communications on this subject see Gzot. Mac. 1881, pp. 34, 
491, and 334, and 1882, p. 540. 
