Vill PREFACE. 
Author, during his partial recovery ; this explains the reason 
why they do not agree in several instances with those which 
are generally quoted as his names of the shells. 
The MS. has been printed exactly as it was left by the 
Author, except leaving out certain repetitions or mere transla- 
tions of the characters of some of the Bivalves. 
I have given the three dedications which I found in the MS. 
It is probable that they were intended to be prefixed to the 
three portions of the Work, the first to the Ascidians, the 
second to the Cephalopods and Gasteropods, and the latter to 
the Bivalves. 
~~ 
After the most careful search, I have not been able to re- 
cover the descriptions of the species which occur between the 
printed and the MS. portion; they were probably in type 
when the Work was interrupted, and the MS. mislaid; but in 
order to render the subject complete, I have given from another 
MS. of Dr. Leach, a list of the species omitted. 
JOHN EDWARD GRAY. 
British Museum, 
February 12, 1852. 
