156 
PKOCEEDINGS OE SOCIETIES. 
which is now before us, and of which a careful drawing has been 
made* exhibits the casts of both valves of Posidonia Becheri attached 
symmetrically by the oblique hinge characteristic of that genus, their 
position being exactly that which we have hitherto so much desired to 
see, and they are each marked by the regular concentric ridges which 
form an invariable feature in the identification of Posidonia. 
The genus has been so long established, and its characters are so 
commonly known, that it will, of course, be unnecessary to enter upon 
any formal paleeontological details in furtherance of the inquiry before us. 
It will in this case be observed that the shell is nearly orbicular, dif¬ 
fering in this respect from the obliquely elongated form which it usually 
presents, but we are aware that the species is subject to considerable 
variations in outline, as a series of specimens would exhibit gradations 
from the extreme of obliquity to such remarkably circular forms as the 
example before us affords. 
Amongst numerous other specimens which have been collected at 
Loughshinny, there is only one in which a faint trace of two valves can 
be discerned; so that it would appear that the occurrence of such a fossil 
as that before us is an unusual circumstance. 
I may, however, observe that the consideration which has heretofore 
been chiefly relied on, in justification of the doubt regarding the affinities 
of Posidonia, namely, the absence of the valves in apposition, has had 
an importance attached to it which, it appears to me, was not sufficiently 
borne out by such a circumstance; as any one, even the most cursory 
observer, cannot fail to have remarked that amongst the numerous 
variety of shells which lie scattered on most sea-shores, comparatively 
few are found with both valves attached, either closed or open, and 
the longer such remains lie exposed to the vicissitudes to which, from 
various atmospheric and mechanical causes, they are subject, the chances 
in favour of ligamentary attachment will be continually lessening. Of 
course I am aware that the force of a consideration apparently so simple 
is very much increased by the light which such a discovery as that 
before us affords; very simple circumstances, from which correct infe¬ 
rences might be drawn, being often overlooked in the absence of tangible 
facts. A similar remark applies to fossil bivalves generally, as in com¬ 
paratively few instances do we find both valves attached, while the oc¬ 
currence of single valves is most commonly to be observed. 
As an exemplification, I might select the case of the genus Aviculo- 
pecten, which in its numerous species rarely presents to our view more 
than a single valve, though (as has been completely overlooked in the 
case of Posidonia) the right and left valve of the same species unattached 
are frequently found to occur, thus enabling us to arrive at as certain a 
conclusion (and in both cases equally) as if we were in possession of the 
accidental fact of apposition. 
In addition to the collection which I have made, the President Jias 
enabled me to exhibit specimens obtained by him from the same locality, 
* See Plate XVIII., Fig. l. 
