122 
\ 
some period subjected to a great degree of intestine mo- 
tion while under so much confinement that the pebbles 
could not move from their situations, and thus protected 
the smaller shells that fell into the hollows between 
them, while they were wearing away such as came in 
contact with them. Some of the older shells are worm- 
eaten, but no Serpula or Coraline has been found attach- 
ed to them. 
OSTREA edulina? 
TAB. CCCLXXXVIll. — Figs. 3 and 4. 
Spec. Char. Suborbicular or obovate; lower 
valve rather concave, composed of undula 
ting laminae ; the other smooth and flat ; 
beak pointed, curved. 
Ostrea edulina? Lam. Hist . Nat. 6. pt. 1. 218. 
A. smaller and much lighter shell than the last, although 
often approaching it in form ; the great smoothness of 
the flat valve and the want of straight lines on the sides 
of the beaks will serve as distinguishing marks ; when 
several are grouped together, they are apt to be much 
elongated, and then come very uear to O. tener, Min. Con. 
252, of which however both valves are free from undula- 
tions. 
Very abundant in beds and layers alternating with 
fresh water shells, among dark coloured clay, forming a 
thick stratum free from pebbles immediately below that 
in which the last species occurs in the Sand-pit near 
Woolwich. 
It is very probable that several species are included un- 
der the name edulina by Lamarck of which this may be 
one ; but it is doubtful and it will ever be difficult to ascer- 
tain the point, the shells themselves are so variable. 
