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 

 Verneuil, Russie, vol. ii. p. 158. 



