328 GINGLYMACONCHA. UNIONIDZ. 
a dwarf variety, which in its young state cannot be distinguished 
from what is generally denominated M. anatinus. 
The most common variety occurs in most of the canals and 
stagnant waters around London, where it is very abundantly 
found, but rarely exceeds five inches in length; in Norfolk, 
about Norwich, it is frequently found six inches and a half in 
length, and three and a half in height. The largest specimens 
that I have seen measure nearly eight inches in length, three 
and a half in height, and two and a half in width, and were 
communicated to me by Sir Joseph Banks, who discovered 
them in a pond in the front of his house at Spring Grove, that 
had for many years remained undisturbed. 
The second or thick variety was observed by Miss Benett, 
in a brook near Hillground Hatches, parish of Tisbury in Wilt- 
shire : this is proportionally higher than the first variety ; the 
under margin is very much indented or excavated ; the epi- 
dermis is black, and the umbones very much corroded ; in the 
young state it cannot be distinguished from the common va- 
riety : the largest specimen that I received measured five inches 
and a half in length, and three and a half in height. 
The third variety, Mytilus Avonensis of Montague, occurs in 
the river Mole, near Cobham in Surrey; in the Brent, near 
Hendon in Middlesex; in a brook at Castle Carey, and in 
Bishopstrow river, near Warminster, Wiltshire, abundantly ; 
but rarely exceeds two inches and a half in length ; in the Avon 
at Lackham, it acquires the length of three inches and a half; 
and in the Tay, four inches. This variety is found, with all 
the intermediate varieties that connect it with the first variety, 
in the New River, in the Thames, in the Mole ; in ponds, near 
Boreham, Wiltshire ; in the Avon above and at Bath; in the 
Cam; in Norfolk, in a stream, near Erpingham in the latter 
county; in streams near Newcastle, and round Edinburgh ; 
and in the Wye at Hereford. 
All the above-mentioned varieties, which are in the collection 
I presented to the British Museum at different times, pass 
into each other imperceptibly, and all perfectly agree with 
each other in the young state, when the epidermis is of a pale 
luteous green colour, with very large wing-like processes on 
