Fossil Organic Uemams. 33 
tfalogist who has long attended to this matter, M. Brongniart, 
u there exists a kind of transition between the marine limestone* 
and the fresh water gypsum which follows this limestone ; and 
that these two formations do not present* in an abrupt man- 
ner, the separation which is seen, in the same places, between the 
chalk and coarse limestone ; that is to say, between two marine 
formations. It cannot be doubted,” adds the same observer, 
u that the first beds of gypsum have been deposited in a liquid 
analogous to the sea, while the following ones have been depo- 
sited in a liquid analogous to fresh water.” ( Gebgr, Min., p. 168. 
and 193 ) 
In announcing the motives which prevent me from general- 
izing a terminology founded upon the contrast between fresh- 
water productions and marine ones* I am far from disputing the 
existence of a fresh-water formation, superior to all the other ter- 
tiary formations, and which contains only bulimse, limnese, cy- 
clostomata and potamides. .Recent observations have demon- 
strated, that this formation is more general than was at first be- 
lieved. It is a new and last term to add to the geognostical se- 
ries. We are indebted for the knowledge of this fresh-water 
limestone, to the useful labours of M. Brongniart. The pheno- 
mena which the fresh-Water formations present, whose existence 
was formerly known only by the tufas of Thuringia, and by the 
Travertine of the plains of Rome, (Reuss, Geogn * v. ii. p. 648. ; 
Buch, Geog. Beob. v. ii. p. 21.-30.), are conformable, in the 
most satisfying manner, to the admirable laws discovered by M. 
Cuvier, in the position of the bones of viviparous quadrupeds. 
(Brongniart* Annates du Museum * vol. xv p. 857.-581.; Cu- 
vier, Reck, sur les Ossemens fossiles , vol. i. p. 54. 
The distinction between fluviatile and marine shells is the 
object of very delicate inquiries ; for it may happen, when the 
remains of organic bodies are, with difficulty, detached from the 
siliceous limestone in which they are contained, that ampullarige 
may be confounded with naticse, potamides with cerithia, and so 
forth. In the family of Conchse, the cyclades and cyrenae, the 
venuses and lucinse, cannot be separated with certainty, but by 
the examination of the teeth of the hinge. The work which M. 
de Ferussac has undertaken, on the terrestrial and fluviatile shells, 
VOL. IX. NO. 17 . JULY 1828 , 
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