208 LAND AND FRESH-WATER MOLLUSKS. 
Nevertheless, Mr. Clark suggests the following 
solution:—- 
“ It is difficult to conceive how they can with 
safety come to the surface to breathe pure air, 
as, during the rainy seasons, the currents would 
probably drive such light and delicate animals 
down the stream, and cause them to perish, if 
they ventured to quit their moorings; they must, 
therefore, remain at anchor, unless they have the 
power, when they wish to breathe the pure air, 
of veering out a filamentary cable, by which they 
can withdraw again, after respiration, to their 
original site.” 
From their small size and sedentary habits the 
river limpets are not frequent denizens of our 
aquariums. The jelly-like egg masses of this 
species are attached to stones, and are ovate; 
they contain from four to eight eggs placed in a 
row. An annelid, Matzia heterodactyla , Vogt, 
lives in the mantle cavity of this species. 
Ancylus (Velletia) oblonous —{the Oblong 
or Lake Limpet ) (PI. XI., fig. 139).— This 
species, often termed Ancylus lacustris , or Vel¬ 
letia lacustris, is at once to be distinguished 
from the only other British species, A. fluvia- 
tilisy by its oblong shell, and by the form of 
the apex, which is twisted to the right, the 
shell being thus dextral. Dr. Gray regarded 
this difference of generic value, and consti- 
