114 Dr. F. A. Bather— Fossil Annelid Burrows. 
From these considerations it appears that the differential motion of 
glaciers is partly the result of viscous shear between adjacent 
granules; partly of viscous shear along planes at right angles to 
the optic axes of the crystalline granules; partly of the plastic 
shearing into separate pieces of the granules and to the viscosity 
imparted to the general mass by the growth of one crystal at the 
expense of another. It is to this latter giving way of the points of 
attachment between granule and granule, owing to molecular move- 
ments at their interfaces, that the general viscosity of the glacier 
depends. The deformation of the granules by stress and the formation 
of extensive shear planes by the breaking up of granules are secondary 
effects. 
EXPLANATION OF PLATE XIII. 
Fig. 1 shows a portion of the ice surface, consisting of several granules, in the 
Upper Cave of the Rhone Glacier, magnified 2°5 diam. 
Fig. 2 shows the surface of a large granule in the same cave, magnified 2°5 diam. 
Fig. 3 shows an enlargement of a portion of the surface of the granule in Fig. 2, 
enlarged to 10°8 diam. 
ViI.—Some Fossrz ANNELID Borrows.! 
By Dr. F, A. Barner, M.A., F.R.S. 
N the little note on ‘‘ Fossil Representatives of the Lithodomous 
Worm Polydora” which appeared in the GrotocicaL Magazine 
for March, 1909 (pp. 108-10), it is said that, ‘‘so far as I can 
ascertain, this genus has not hitherto been recorded in a fossil state.” 
It has since come to my knowledge that I was not the first so to 
record it, and I therefore ask permission to make the necessary 
emendation. 
In March, 1908 (Buil. Soc. géol. France, ser. 4, vol. vil, pp. 361-70, 
pl. xii), Professor Henri Douvillé published a paper on ‘‘ Perforations 
d’Annélides’”’, in which he figured a surface of Jurassic rock, from 
near the fort of Arrabida in Portugal, penetrated by burrows 
characteristic of Polydora. Since these Jurassic rocks are covered 
by others of Helvetian age, the borings cannot be of later date than 
Middle Miocene. These perforations, which were first observed by 
Mr. P. Choffat (December, 1906, Bull. Soc. géol. France, ser. 4, vol. vi, 
p- 237), are about twice as large as those of Polydora ciliata, and are 
therefore compared by Professor Douvillé with those of P. hoplura 
Claparéde. He adds: ‘On pourrait réserver a ces perforations la 
terminaison ztes et les désigner sous le nom de Polydorites.”’ Since no 
distinction can be drawn between the borings thus named Polydorites 
and those made by Polydora, it appears that ‘ Polydorites’ is not 
intended as an independent generic name, but merely as a brief way of 
writing ‘fossil traces of Polydora’. 
Polydora hoplura is a British species, though not so common as 
P. ciliata. Others placed in the British list by Professor W. C. 
M‘Intosh (February, 1909, Ann. Mag. Nat. Hist., ser. 8, vol. iii, 
1 Published by permission of the Trustees of the British Museum. 
