224 
from P. parva in its greater width, and the less degree 
of curvature of the posterior sinus ; it is also smoother, 
and in some respects resembles P. candida. 
The P. Campechiensis, referred to by Parkinson, at 
p- 198, Vol. ILI. is probably distinct. 
I am indebted to Mr. Parkinson for the use of the spe- 
cimens of this tender shell, found, rarely perfect, in 
Crag. 
