GEOLOGICAL SOCIETY OE DUBLIN. 
155 
ferous locality, and as many of these minerals appear to be of a very 
recent origin, it may happen that some deposits of phosphatic minerals 
may exist in the locality. 
Dr. E. Percival Wright read the following paper:— 
ON THE OCCURRENCE OF A RARE FORM OF POSIDONIA BECHERI IN THE CALP 
OF RUSH, COUNTY OF DUBLIN, AND OF POSIDONIA LATERALIS IN THE CAR¬ 
BONIFEROUS SLATE OF KIN SALE, COUNTY OF CORK. BY SIR RICHARD 
GRIFFITH, BART., F. G. S. 
Paleontologists have hitherto been unable, with any certainty, to de¬ 
cide whether the genus. Posidonia, or Posidonomya, should be regarded 
as a Conchifer, or as the internal plate of an animal allied to Aplysia. 
The prevailing opinion, no donbt, tended to the belief that it was a 
bivalve shell, with affinities approaching to the family Aviculidse, but as 
no example of a specimen exhibiting both valves, either closed or open, 
had been discovered, it was impossible to arrive at any satisfactory con¬ 
clusion. 
A very slight review of what has been written on this subject will 
be sufficient to show the importance of the present communication. 
Professor Phillips (“ Pal. Eos.,” p. 44), in speaking of the genus 
Posidonia, says :—“It is remarkable that no case has come under my 
notice of a specimen in which the opposite valves were in exactly sym¬ 
metrical apposition.” And he further remarks that Bronn, the founder 
of the genus, gives a drawing (“ Lethsea Geognostica,” PI. ii. Eig. 175), 
which implies that he has seen such a one. But upon referring to 
Bronn’s figure, it would appear to have escaped Mr. Phillips’ notice that 
it is only given as an ideal side view (Ideale seiten ansicht), besides 
affording an idea of convexity, which we do not find warranted by the 
fact, making every allowance for compression. 
Taking it for granted that the two valves of Posidonia have not been 
hitherto observed in apposition, I feel gratified in being able to bring 
forward a certainty in opposition to the ingenious conjecture of M. 
Deshayes, namely, that the fossils in question were thin, single plates, 
of the nature of the gill-cover of Aplysia,—a theory to which some pro¬ 
bability has been attached by subsequent writers, from the resemblance 
which, though no doubt considerable, is yet quite insufficient to support 
a theory. Through the kindness of Dr. Earran, I have been enabled to 
compare one of these plates with the fossil; but the former is wholly 
deficient with respect to the aviculiform sub-auriculation, which is always 
displayed by the latter; the remaining features, such as the apparent 
umbo, and irregular concentric w'rinkles, teaching us that mere general 
resemblances are frequently very delusive grounds of assumption, not 
only in this case, but in any other. 
. In the progress of my geological examination of the middle lime¬ 
stone, or calp rocks of Bush, near Dublin, at a place called Lough- 
shinny, my attention was attracted to a very clearly-marked Posidonia 
BecJieri, which I further found would afford conclusive evidence in 
reference to the settlement of the point in question. This specimen, 
