60 EOCENE MOLLUSCA. 
fresh-water Limnee in company with salt-water animals, as previously noticed by Mr. 
James Sowerby in his ‘Mineral Conchology.’ Iam inclined to think the water where 
these animals were deposited was quite fresh, although Limnaz have been found living 
in saline marshes; the reptile was probably stopped by a bend of the river or by 
some impediment in the stream; the presence of a large number of the scales of 
Lepidosteus in this layer of sand indicates the greater probability of its having been more 
of a fresh-water deposit than brackish. 
D. Brardii is given by Mr. Prestwich in his list of the organic remains of the 
Woolwich and Reading series (Jour. Geol. Soc., 1847, p. 117); he says, however, at 
p. 120 (/)—‘this shell is here in too imperfect a state to admit of a positive 
determination.” 
Two specimens, with the name of Mytilus Brardii, were obligingly sent to me 
many years ago by the Comte du Chastel, and to these was attached the locality of 
Bordeaux. I presume they are what have since been called Dreissena Basterotti. 
On a close comparison with the British fossil, these specimens present the following 
differences: they are rather more pointed, less curved at the umbo, and they have an 
obtuse elevation or sort of ridge at the most inflated part, which has probably caused 
them to be called “ subcarinated.” Whether these slight differences be permanent or 
whether they are such as will constitute a specific distinction, the few specimens of the 
Bordeaux shell that I have seen will not permit me to give a decided opinion. 
MYTILUS. JZinneus, 1758. 
Generic Character. Shell equivalve, wedge-shaped, acuminated, rounded on _ the 
siphonal side; umbones pointed, terminal; hinge-teeth few, minute, sometimes obsolete ; 
muscular impression at the umbo small and narrow; pedal impressions two in each 
valve ; connexus bipartite, marginal ; interior of shell nacreous. 
Animal elongate, with the lobes of the mantle partly fringed, plain in the anal region, 
and slightly projecting ; disconnected except where there is a separation for the siphonal 
openings ; adductor muscles very unequal in size ; foot cylindrical, furmished with a gland 
and groove; byssus strong and coarse. 
This genus is generally marine, though sometimes estuary in its habits, and, I believe, 
M. edulis is known occasionally to live with Limnzee, &c., when the water is fresh. This 
common edible mussel frequents mud banks, and, as it is well known, is more often found 
where it has been deserted by the retiring tide; others mhabit the sea at considerable 
depths. In the recent state the genus numbers probably fifty species, and these have a 
very extensive geographical range, taking in the whole circumference of the globe. The 
generality of the species are extra-tropical. Fossil species are abundant, and are said to 
be found in rocks of the Permian age. 
