rN) 
PACHYMYA Gigas. 
TAB. DIV. and DV. 
Spec. CHAR.....- 
A a1nzoseE heavy shell, ahove twice as wide as long; the 
thickness also exceeds thelength. Itis slightly curved, 
with parallel edges. The anterior extremity is small, 
rounded ; the posterior, rather truncated: they are both 
close. The valves are rendered obliquely boat-shaped by 
a ridge that runs from the beaks to the lower extremity 
of the posterior edge; the surface is smooth except to- 
wards the margin, where it is formed of the imbricated 
edges of lamine. 
We are indebted to the zeal and kindness of H.'T. De 
la Beche, Esq. for the opportunity of representing this 
extraordinary shell: itis from the Chalk with quartzose 
grains at Dowlands, the lowest part of the Chalk for- 
mation in the vicinity of Lyme Regis. 
The generic name suggested by Mr. De la Beche ex- 
presses a slight resemblance which the shell has to some 
species of the Linnean genus Mya, 
