204 HISTORICAL PALEONTOLOGY. 
is represented by an immense number of specific forms, some 
of which attain very large dimensions. In the Eocene strata 
of the Paris basin alone, nearly one hundred 
and fifty species of this genus have been 
detected. The more strictly fresh - water 
deposits of the Eocene period have also 
yielded numerous remains of Univalves such 
as are now proper to rivers and lakes, to- 
gether with the shells of true Land-snails. 
Amongst these may be mentioned numerous 
species of Limnea (fig. 220), Physa (fig. 221), 
Melania, Paludina, Planorbis, Helix, Buli- 
mus, and Cyclostoma ‘fig. 222). 
With regard to the Cephalopods, the chief 
point to be noticed is, that all the beautiful 
: and complex forms which peculiarly char- 
Fig. 219.—Cerithi- acterised the Cretaceous period have here 
we eee Fo disappeared. We no longer meet with a 
single example of the Turrilite, the Baculite, 
the Hamite, the Scaphite, or the Ammonite. The only ex- 
ception to this statement is the occurrence of one species 
Fig. 220.—Limnea Fig. 221.—Physa Fig. 222.—Cyclostoma 
pyramidalis. Eocene. coluninaris. Eocene. Arnoudit. Eocene. 
of Ammonite in the so-called “ Lignitic Formation” of North 
America ; but the beds containing this may possibly be rather 
referable to the Cretaceous—and this exception does not 
affect the fact that the Ammonitide, as a family, had be- 
come extinct before the Eocene strata were deposited. The | 
ancient genus /Vaz/z/us still survives, the sole representative of 
the once mighty order of the Tetrabranchiate Cephalopods. 
In the order of the Dzbranchiates, we have a like phenomenon 
to observe in the total extinction of the great family of the 
“‘ Belemnites.” No form referable to this group has hitherto 
