File:The Quarterly journal of the Geological Society of London (13205635985).jpg

78
PROCEEDINGS OF THE GEOLOGICAL SOCIETY. .DeC. %
win are very numerous ; a list of them has been given by Professor
Phillips in the ' Palaeozoic Fossils of Cornwall/ &c., p. 142 : the
most common is the Spirifer disjunctus.
For convenience of comparison I shall draw most of my illustra-
tions from the Spirifer giganteus and S. disjunctus of Sowerby,
which I believe to be one species *. At South Petherwin the speci-
mens of *S'. disjunctus found in the limestones are much smaller than
those in the slates, from which we learn that lime was not favourable
to the growth of the species, and we need not be surprised that it
reached a larger size at Tintagel, where no lime is present ; but the
difference in the size of the South Petherwin and Tintagel specimens
appears greater than it really is, owing to the greater distortion of
all the Tintagel shells. Moreover, a careful comparison of the series
of beds in the two localities convinced me that the Tintagel shells
come from the same bed as those of South Petherwin. To establish
a term of comparison with the distorted specimens shortly to be
described, we require the original form of this species, which will be
found in fig. 1, copied and slightly restored from Mr. Sowerby 's
figure of Spirifer disjunctus just referred to. Fig. 2 gives a sup-
posed outline of the section of the same shell, restored from several
specimens. These figures give the following proportions : A B the
length of the shell, measured from the hinge-line to the bottom of
the mesial fold, is half the length of the hinge-line C D ; the thick-
ness of the shell E F is two-fifths of the length of the hinge-line C D.
The same letters will have the same signification in all the illus-
trations. The proportions here given are not always true, but they
are the best approximations we can get, and as such are used in the
calculations which follow.
Besides other causes, partly unknown, the amount of distortion
appears to depend upon the angle at which the cleavage cuts the
bedding ; the distortion being greatest where the angle between
them is the least. It has also had a very different effect upon flat and

• See Sowerby, Trans, of Geol. Soc. 2nd Series, vol. v. description of figs. 12
  

and 13, pi. 54. De Koninck, Description des Fossiles de Belgique, p. 254. De