GEOLOGICAL SOCIETY OE DUBLIN. 
157 
in one of which a portion of the shell itself has heen preserved, and, 
notwithstanding its thinness, it does not exactly justify the anticipation 
we might have formed relative to its membraneous or extremely fragile 
character, its thickness being not inaptly compared to that of an egg¬ 
shell ; and when we remember that these fossils are, for the most part, 
mere impressions, the fact of the ridges of different individuals crossing 
each other in various directions will occasion no surprise; nor can we 
necessarily conclude that the shell was flexible on that account: on the 
contrary, we are now*aware that it was of a firm and brittle texture. 
The President has also kindly favoured me with another specimen 
which, though it would have been insufficient without the aid of my 
more perfect fossil to determine the inquiry satisfactorily, is yet very 
valuable, as affording a repetition of the fact we are considering; and 
I am happy to find that Dr. Carte, whose attention has been directed to 
the subject, has lately succeeded in procuring a third*' example in which 
the bivalve structure can be clearly traced; and though, in common 
with many other lamellibranchiate fossils, the closed valves in appo¬ 
sition have not been hitherto observed, yet I should say that a positive 
statement as to the non-existence of such a case would be a very unsafe 
position, as we are not in a condition to say that the opposite valve may 
not be present in many cases, though concealed in the adhesive matrix 
of surrounding shale. 
Little need be said further as to the shell itself, except that it were 
to be wished that some more satisfactory’mode of defining the present 
species (if not of the whole genus) existed, as one sometimes feels a 
difficulty in ascribing identity, or the contrary, to such varied forms as 
frequently present themselves; and it may be worth our notice how far 
some of the species at least, which at present are regarded as belonging to 
the genus Inoceramus, may not rather form a portion of the genus Posi- 
donia, a consideration probably of some importance with respect to the 
geological bearings of the subject. 
In the strata alternating with the fine-grained dark shales which 
contain Posidonia, as well as in those latter, we find the carboniferous 
fossils, Fenestella , Productus co'ncinnus , Goniatites reticulatus and spi¬ 
ralis, the last-mentioned (which has been frequently supposed to be an 
Orbicula) being obtained perhaps in a better state of preservation and 
of a larger size in the shales of Push than in those of any other locality 
in Ireland. Yegetable remains are also of frequent occurrence in the 
Goniatite beds, and these remains are undoubtedly identical with those 
occurring at the Naul and Clontarf, which have been described and 
figured in a former Number of our Journal by my friend Professor 
Haughton. Posidonia membranacea is likewise very abundant in this 
locality, but it is difficult to procure a good cabinet specimen of the 
fossil, owing to the rapid decomposition to which the external surface 
* Mr. Hargrave, one of Mr. Haughton’s pupils, lias recently obtained another speci¬ 
men, showing both valves, from the neighbourhood of Rush. See Plate XVIII., Fig. 2. 
VOL. V.-PK0C. SOC. Y 
