Illustrative Group of Shells : — 
Univalves. 
a, Limnéa, I. Wight, Fresh-water. 
6, Planorbis, Isle of Wight, Fresh- 
water. 
c, Murex, or rather Fusus, contra- 
g, Pholadomya (Cardita), Glouces- 
tershire, Lias. 
h, Terebratula, Suffolk, Crag. 
Complicated Bivalves. 
rius; Suffolk, Crag. 2, Spirifer, Derbyshire, Mountain 
d, Rostellaria macréptera, Barton, limestone. 
London clay. k, Pentamerus, Ludlow, Transition 
; limestone. 
Bivalves. 
. é 4 » . / ) » W977 
ec, Unio, Hordwell Cliff, Fresh- Chambered or Multilocular Univalves. 
water (lower). 
f, Cyclas, Woolwich, Freshwater 
(lower), or Plastic clay. 
7, Ammonifes, Folkstone, Gault. 
m, Baculites, Chalk and Chalk 
marl. 
When the shellfish that inhabit our ocean are compared 
with the fossil tribes, essential specific differences are per- 
ceived; and these differences become more striking as we 
recede from the latest formations. In our crag and fresh- 
water beds some species may be discovered which possess a 
strong similarity, if not absolute identity, with those living in 
our lakes and seas. Even here, the identity is maintained but 
by a limited number, which are intermixed with numerous 
others that have no recent analogues. Investigations in fossil 
conchology lead, therefore, to one result ; that, with the incon- 
siderable exceptions that have been stated, the species have 
not been perpetuated to our times. 
We have seen that fossil remains of peculiar character oc- 
cupy certain parts of the grand series of conchiferous deposits ; 
but this transition is seldom abrupt. The same organic remains 
Vou. Lik— No. I3. U 
