ae INTRODUCTION. 
more than the collector, who can glance at a plate where 
the shell is figured. Some ludicrous results of attempting 
to realize form from description may be seen in the draw- 
ings of the old naturalist Gesner, who depicts elephants, 
whales, besides other beings more fearful and wonderful 
still, from the descriptions of people who had seen them, 
or professed to have done so. 
Shells recently admitted into the British List. 
Within the last few years several shells have made their 
appearance in these islands. 
Some of these, whose introduction from foreign parts is 
fairly established, have been admitted to rank with indi- 
genous species. These are P. dilatatus, which came in 
American cotton bales, and 7. Maugei, which is shown to 
have been introduced with continental plants sent to Bristol. 
Both of these species have made themselves at home, and 
are spreading. 
Others, known hitherto only as foreign, and whose 
method of introduction is obscure, have been admitted by 
some as British species. Among these are Clausilia parvula 
and Helia villosa. 
Zonites glaber, Vertigo Lilljeborgu, and V. Moulinsiana, 
though they had escaped notice till quite recently, can 
hardly have been introduced by the ordinary methods, and 
may fairly claim to be indigenous. 
A few long-established British species, whose introduc- 
