DREISSENSIIDA. 59 
South. 
Bridgwater ; Crotch. 
River Yeo at Yeovil; J. Ponsonby. 
Var. cerina, Colbeau. 
There are specimens in the Bristol Museum from the 
Bath Canal, labelled “ This rare variety of N. fluviatilis 
was discovered by Miss F. M. Hele, of Bristol, in 1882.” 
DREISSENSIIDE. 
DREISSENSIA POLYMORPHA, Pallas. 
An alien species, supposed to have been introduced into this 
country in or about the year 1824 with timber from Russia. 
Mr. Hugh Strickland, in a short paper on the naturalization 
of Dreissena polymorpha in Great Britain, contributed to the 
Magazine of Natural History in 1838 (vol. 11, new series), re- 
marked that it had “lately been planted by Mr. Stuchbury, of 
Bristol, in some waters near that place.” He considered that 
“it appears desirable to record these particulars, because it 
may interest some of our field-naturalists to watch the gradual 
spread of this species over the kingdom. Its propagation is so 
astonishingly rapid, that it will probably become, in a few 
years, one of our commonest British shells.” Ten years later 
it had been reported from two counties in Scotland and thirteen 
in England. In the census list of British non-marine mollusca, 
published in 1902, it is given under twenty-five English counties 
and four Scotch. 
Its absence from the Dumball Island deposit already alluded 
to (p. xiii), is of some interest. Mr. Bolton writes me that “ at 
the time when the actual deposit was being made, Dumball 
Island was practically part of Somerset, only a narrow shallow 
channel separating it from the Somerset shore, whilst a deep 
channel, available for ships, separated it from the Gloucester 
shore. This is less than 100 years ago. The deep channel 
afterwards silted up entirely, and Dumball Island became at- 
tached to the Gloucester shore, whilst the shallow channel 
deepened in a similar fashion, and is now the only channel of 
the river.” If the silting up took place after the ‘ planting’ of 
D. polymorpha near Bristol in 1838, it is very probable that 
this species would have been found there. When was it first 
observed at Bath? It seems to have been unknown in Wilts 
prior to the sixties. “The Dreissena is perhaps better fitted 
for dissemination by man and subsequent establishment than 
