INTRODUCTION. xill 
apparent head*, they have nevertheless those most essen- 
tial parts of the head, namely, the mouth, furnished with 
double bilobate lips, and an cesophagus. 
I must say a little more in my defence before I conclude 
this Introduction, respecting the minute divisions I have 
made in the new genera which I have given in this Work. 
I have three species of Physa, two of Nauta, five of Gulnaria 
from Switzerland ; of Pomatia I have seen five species, 
besides that inhabitmg England ; one from Smyrna, given 
to me by Dr. Merion, which is now in the British Museum ; 
three species in the collection of Lamarck, and one is in 
the Collection of Baron de Férussac: that from Smyrna 
Dr. Merion assures me had a calcareous epiphragm ; and 
from the similar structure of the peritrema in all the species, 
I have no doubt but that they all have a similar one. 
MOLLUSCA. 
Animalia mollia, nuda; aut testA una interna vel externa ; 
aut testis duabus externis induta. 
Animals with soft bodies, naked; or furnished either 
with one internal or external shell; or with two external 
shells. 
General Remarks on the Mollusca. 
All the animals of this group have neither a skeleton 
nor a vertebral canal. 
Their nervous system does not unite into a spinal mar- 
row, but consists merely of medullary masses dispersed 
* Cuvier admits them to have a head, ‘ Régne Animal,’ vol. ii. p. 408 ; 
“On ne lui voit déaotable qu’une double rangée des franges autour du 
manteau, laquelle n’a ses lebes unis qu’au-dessus de la téte.” 
b 
