4 
68 Proceedings of Philosophical Societies. [Jury, 
the strata is regular and slightly inclined, nor have any sub- 
terraneous disturbances operated to any important degree, to 
affect the form of the vallies. 
The mass of chalk which at Beer Head composes the entire 
thickness of the cliff, gradually rises westwards, with a con- 
tinual diminution of its upper surface, until after becoming 
more and more thin it finds in Dunscombe hill its western 
boundary. Beyond this boundary, on the top of all the highest 
table lands and insulated summits, from the ridges that encircle 
the vales of Sidmouth and Honiton, to the summits of Black- 
down and even Haldon west of the Exe, angular chalk fints 
are found. Similar chalk flints are found on the summits 
of green sand that encircle the vallies of Charmouth and Ax- 
minster; and large insulated masses of chalk itself are found 
along the coast from Lyme, nearly to Sidmouth, and in the 
interior at Wideworthy, Membury, Whitestanton, and Chard, 
at the distances of from 10 to 30 miles from the escarpment of 
the chalk. These facts concur to show, that there was a time 
when the chalk covered all those spaces on which the flints are 
now found, and that it probably formed a continuous stratum, 
from its present termination in Dorsetshire, to Haldon west of 
Exeter, 
Similar observations are made by the author concerning the 
green sand, and similar inferences are drawn from them as to 
the former continuity and subsequent excavation of its strata. 
May 3.—A Paper was read, entitled “ Additional Notices 
on the Fossil Genera Icthyosaurus and Plesiosaurus,” by the 
Rey. W. Conybeare, M.G.S. 
This paper consists principally of anatomical details not sus- 
~ ceptible of abridgment. It fills up the outline of the history 
~of the fossil genera Icthyosaurus and Plesiosaurus, sketched 
i a preceding communication published in the 5th vol. Trans- 
actions Geological Society, and establishes five different spe- 
cies of Icthyosaurus, principally distinguished by the form of 
their teeth. A particular account of the dentition of this 
genus is given by the author, from which it appears that it re- 
sembles that of the crocodile in the general form of the teeth, 
and the general mode in which the secondary teeth replace the 
first set; but differs in the circumstance, that the latter teeth 
become in advanced age, completely solid, by the ossification 
of the pulpy matter filling the interior cavity, which in the 
crocodile always remains hollow, a constant developement of 
successive series of new teeth taking place in the latter animal. 
In this point the dentition of the Icthyosaurus agrees with the 
other genera of the Saurian order, to which the term lacertian 
may most strictly be applied. Bi 
The comparative analogies of structure exhibited by the 
Icthyosaurus to both these branches of the Saurian order, are 
examined and illustrated in detail in the present communica- 
