SPIRIFERA. 25 



Sp. gigantea of Sowerby and Phillips, without the slightest change of character, so that 

 many of the specimens at Petherwin and Tintagel could not be referred by a conscientious 

 observer to the one species in preference to the other. The character supposed by Phillips 

 to distinguish Sp. gigantea from Sp. disjuncta, viz., the mesial fold being ' indistinctly 

 furrowed or nearly smooth,' is negatived by Mr. Sowerby's original figures, by his type- 

 specimens, and the greater number of others which I have seen. Mr. Sowerby has himself 

 suggested the possible identity of his Sp. disjuncta and Sp. gigantea ; and De Koninck and 

 De Verneuil suspected the possible union of the four so-called species here united. Cleavage 

 and pressure have produced the most singular contortions in specimens of this and other 

 species ; some are completely flattened or greatly elongated, while others are made to 

 assume much more than their natural width ; some, again, are bent and twisted to the one 

 or other side, and completely put out of shape. 1 



When describing Sp. calcarata, Sowerby states that his shell (of which the charac- 

 ters are taken from a single internal cast) "approaches closely to Sp. attenuata, M.C. ; 

 but distinguished by the sudden contraction of the sides and the very slight elevation of 

 the front. Still we have some doubts of its being distinct from the many varieties of that 

 species." This leads me to observe that I possess two or three specimens of Sp. striata, 

 var. attenuata, with simple lateral ribs, identically similar in every respect to the Devonian 

 example (PI. V, fig. 4 ); but it must be remembered that in the greater number of specimens 

 of Sp. striata the ribs upon the lateral portions of the valve augment very much in number 

 by the intercalation of additional ribs at various distances from the beak and umbone, 

 while in Sp. disjuncta they are all simple. It is, therefore, easy by this character alone to 

 distinguish the larger number of specimens of the two species, while we cannot deny that 

 some few can hardly be separated. The character of the ribs in the fold and sinus of the 

 Carboniferous and Devonian species are exactly similar ; and in both we often find a couple 

 of deeper grooves, which sometimes appear to divide the fold into three portions, as is so 

 common in Sp. bisulcata. Sp. disjuncta occurs in the Upper Devonian brown grits of 

 Croyde Bay, seven miles west-north-west of Barum ; in the neighbourhood of Barnstaple, 

 Braunton, &c. ; at South Petherwin, in slates and subordinate limestone ; in chloritic 



1 The reader is referred to a very interesting and important paper "On Slaty Cleavage," by Daniel 

 Sharpe, in the ' Quarterly Journal of the Geological Society,' vol. v, p. 74, 1847, in 'which the author has 

 given an elaborate description and illustration of the many modifications effected on Spirifera disjuncta 

 by cleavage. He also observes, that " in determining the species of fossils of the older forma- 

 tions, great difficulties arise from the distortion of form which they have frequently undergone, which 

 sometimes makes it impossible to ascertain the species or even the genus to which they belong. This 

 distortion varies much in specimens from the same locality, and still more so in those from different 



districts These differences in the angle at which the cleavage intersects the bedding 



appear to have had an influence on the amount of distortion of the organic remains, as these are usually 



most altered in form where the angle of incidence of the planes is the least The distortion 



of the organic remains, in each bed, is in proportion to its cleavage ; in the slates they are extravagantly 

 distorted, in ihe limestones slightly so, while in the soft ochreous beds, in which there is little cleavage, 



the fossils have preserved their form nearly unaltered " &c. 



D 



