Unio distortus (Bean) and Alasmodon vetustus (Brown). 121 
posteally, and transversely sulcated. Its breadth is ;%ths of 
an inch. Adult specimens will probably be found hereafter.’ 
The Great Estuarine Series are Infra-Oxfordian in age, and 
occupy, according to Prof. Judd,* a somewhat analogous 
position in the Jurassic series of the western coast of Scotland 
to that of the series of estuarine strata which contain the famous 
coal seam of Brora in the Eastern Highlands. » 
The specimen on which Prof. Forbes founded his species 
“ staffinensis ’ is, unfortunately, too immature to be of much 
value. Mr. Watson’s example also, being a cast, is too im- 
perfect for accurate specific determination, but the fact of 
it coming from the same beds as the Loch Staffin example, 
leads one to reasonably infer that both are forms of the same 
species. The fact of these beds being looked upon as equiva- 
lent in age to those in Sutherlandshire, and no doubt also to 
those of Yorkshire, makes one inclined to refer these Unios to 
the Gristhorpe form, to which they present so great a likeness. 
Since the above notes were penned, my attention has been 
called to an interesting article by Mr. Bryant Walker, on 
‘The Distribution of Margaritana margaritifera (Linn.) in 
North America,’t in which the author brings forward good 
arguments in favour of an Asiatic origin for M. margaritifera. 
On page 128, he states: ‘ Margaritifera itself is a very ancient 
species, which, through an enormous extent of time, during 
which it has wandered nearly, if not quite around the globe, 
has preserved its peculiar characters and specific identity to 
a remarkable degree. The essential similarity of the species 
as it exists at the present time on the different continents is 
very remarkable, and indicates that its specific characters were 
well established before its long migration was begun. As 
North America has been permanently separated from Asia and 
Europe since the close of the Tertiary period, and the progress 
of the species in its long journey must have necessarily been 
slow, there would seem to be no doubt but that the evolution 
of the species must have long antedated that period, and quite 
possibly may go back even to Cretaceous times.’ 
“Where the species did originate is by no means clear. 
It must have been either in Europe, Asia, or North America. 
That it is an immigrant into Europe is generally conceded. 
Dr. Scharff, in his recent work on European animals (1907, p. 34) 
expresses the opinion that it reached Europe via Greenland and 
Iceland. If so, the inference would be that it originated in 
North America, and from there spread east into Europe, and 
west into Asia. But there are several objections to that theory.’ 
Unfortunately, paleontological evidence is lacking regard- 
* * O:)-G-S:') VOlL Xe xXIV, (1878), p. 722- 
+ ‘ Proc. Malac. Soc.’, vol. IX., pt. i1., June 1910, pp. 126-145, and map. 
git Mar. 1. 
