GEOLOGICAL SOCIETY OE DUBLIN - . 
161 
be observed from the great variety of forms of distortion afforded by 
the Posidonise of Kinsale. 
I have thus shown that Posidoniae occur in the fine-grained, dark- 
coloured shales throughout the entire range of the carboniferous series 
in Ireland, from the base of the marine coal formation* to the upper 
portion of the yellow sandstone group of strata; and it is remarkable 
that in both cases they are accompanied by the fossil, Goniatites strio- 
latus . It only remains to remark, in conclusion, that notwithstand¬ 
ing the prevalence of these fossils in certain groups of strata, they 
would rather seem to afford an indication of mineral conditions and of 
mechanical depositions than be of rigid application in the determination 
of geological subdivisions, as it appears that their presence is dependent 
upon the predominance of argillaceous rather than of calcareous or si¬ 
liceous matter in the bottoms in which they are found; but, however this 
may be, we can safely affirm that they are eminently characteristic of 
the carboniferous series; and it is satisfactory to be able to prove that 
these remains were true lamellibranchiate bivalves, as given in the 
Table of Possils appended to my Geological Map of Ireland. 
NOTE ON THE ALTERATION IN THE EORil OE PO SID ONI A PRODUCED BY CLEA¬ 
VAGE AND THE PRESSURE OE THE SURROUNDING MATRIX. BY THE 
PRESIDENT. 
I have carefully examined the form of Posidonia Becheri from the ellip¬ 
tical rings, in specimens in the Museum of Trinity College, brought from 
Herborn in Nassau. The ratio of axes of the adult shell is 1-38. 
The specimen, Plate XVIII., Pig. 3, from Cony, Drilmkeeran, has 
a ratio of 1 -40, and is identical with the P. Becheri of the Germans. 
Plate XVIII., Pig. 2, represents a variety, with broad, deep annu- 
lations, found in the harder beds of impure shaly limestone at Push, 
county of Dublin. 
Plate XVIII., Pig. 1, represents a nearly circular variety. 
The specimens figured in Plates XVI., XVII., and XIX., are from 
the highly cleaved Carboniferous Slate of Kinsale Head. I believe them 
to be all P. Becheri. 
In the type shell, unfortunately, the elliptical lines of growth are 
oblique to the hinge-line, which is very indefinitely marked, and in con¬ 
sequence it is difficult to say, in the case of the distorted fossils, what the 
original position of the hinge and elliptical lines may have been. I have, 
therefore, found it impossible to calculate numerically the amount of dis¬ 
tortion, as the original position of the shell is unknown, and it was not 
circular. There is, however, a general agreement with the laws of dis¬ 
tortion of fossils, as laid down by me in the “ Phil. Mag.” vol. xii., 1856. 
This is shown by the following series of numbers, which express the 
ratio of axes and angle between cleavage and bedding in the several cases. 
It is to be observed that the major axes of the distorted elliptical 
rings are always parallel to the intersection of cleavage and bedding, 
whatever may have been the position of the rings originally. 
