180 
greatly diminished as to have rendered it difficult, ‘‘ nay impossible,” 
for the animal to have retreated within it. 
In the third place, Mr. Austen observes, that more awkward 
contrivances in progressive motion could not well be conceived than 
the shell of the genus Hamites, on account of its form, supposing it 
to have been external; as well as the thin shells of Orthocera and 
Baculites, on account of their great length. 
He then compares the structure of the Nautilus with that of the 
fossil extinct cephalopods, and from the greater thickness of the 
shell of the Nautilus he infers, that the shells of the extinct genera 
were internal. From the liability of those external shells to be in- 
jured, and thus rendered incapable of performing the office of a float, 
he also infers, that the fossil shells were internal ; and he adds, that 
he has searched in vain for an instance of such a shell having been 
repaired by the animal. On these grounds, therefore, he proposes 
to remove the fossil extinct cephalopeds from the order Tetrabran- 
chiata, to which the Nautilus belongs, and to place them on the 
supposition that they were internal shells, with the Spirula, in the 
order Dibranchiata. 
The memoir to accompany the Second Edition of the Geological 
Map of England and Wales, by George Bellas Greenough, Esq., 
V.P.G.S., was then read. 
Mr. Greenough first points out the changes which have been 
made in the topography of the present edition of his map; he then 
alludes to the alterations in the table of formations, and in the 
boundaries ; and afterwards explains the principles by which he was 
guided in the choice of the pigments used in colouring the map. 
Of the six copper-plates upon which the first edition was en- 
graved, four and the lower portion of the fifth, have been used again ; 
the upper portion of the fifth and the entire sixth are new. 
The southern sheets were drawn originally, in part, at least, from 
those of the Ordnance Survey: for the topography of the northern, 
good authorities are still wanting; and the inaccuracies of the east- 
ern are geologically of little importance: for thus much of the map 
therefore new copper plates were not thought necessary ; not so for 
the remainder. To do justice to the great mass of information which 
is now possessed of Wales, and Siluria more especially, a detailed and 
scrupulously correct drawing was essential. Such a drawing the ad- 
mirable maps which have recently issued from the Ordnance press 
have afforded Mr. Greenough the opportunity of obtaining; and the 
result will, he trusts, be approved not merely by professed geologists, 
but by all who feel an interest in the progress of art, more especially 
when exerted in furtherance of science. 
Having always felt the close connexion which necessarily exists 
between the outward form of a country and its geological struc- 
ture, Mr. Greenough has made it a primary object to represent with 
distinctness and fidelity all undulations of the surface as far as the 
scale of the map would allow. In the first edition more than thir- 
teen hundred heights are distinguished, and in the present more 
