49 
of others where the growth has been interrupted, we 
infer that a slight sinus exists in the lip at the part where 
the keel is formed in the two last wIiqeIs : the mouth 
appears to be oval with an extremely short beak, or 
rather, only a sinus in its lower angle, and an expanded 
outer lip : the keel is placed about one-third down the 
whorl, it is large and rises rather suddenly ; above it 
the grooves before mentioned are sometimes decussated ; 
a small portion of the dark epidermis is remaining upon 
one specimen. 
I am indebted to Lady Burgoine and Miss Beminster 
for the specimens figured of this rare Barton fossil ; I 
consider it to be a fresh water shell, from some one of 
the mixed beds above the London Clay of the Hamp- 
shire coast. 
The decussated grooves had escaped observation 
until I saw them strongly marked in a specimen kindly 
lent me by the Viscount Fitzharris. The apex is com- 
monly destroyed, and holes corroded in various parts of 
its surface, by the action probably of some acid devel- 
oped during the fermentation of vegetable matters, in 
the marsh or at the bottom of the river where the animal 
anciently lived, a circumstance that is not so likely to 
arise in salt water, and has therefore been used to dis- 
tinguish fresh water shells by. 
Figures 1 to 4 represent P, rigidus in different stages 
of growth : figs. 5 and 6 are outlines of the mouth of 
P. ater ; they shew the lip more enlarged than usual in 
the genus, and not reflected or expanded, as in some 
species. 
