138 
NEWER PLIOCENE PERIOD. 
[Ch. XI. 
of recent species. The fresh-water shells which I collected 
near Colle are in a very perfect state, and the colours of the 
Neritinas are peculiarly brilliant. The following six species, 
all of which now inhabit Italy, were identified by M. Deshayes: 
Paludina impura, Neritina fluviatilis, Succinea amphibia, Lim- 
neus auricularis, L. pereger. and Planorbis carinatus. 
Travertins of Rome. — Many of the travertins and calca- 
reous tufas which cap the hills of Rome may also belong to 
the same period. The terrestrial shells inclosed in these masses 
are of the same species as those now abounding in the gardens 
of Rome, and the accompanying aquatic shells are such as are 
found in the streams and lakes of the Campagna. On Mount 
Aventine, the Vatican, and the Capitol, we find abundance of 
vegetable matter, principally reeds encrusted with calcareous 
tufa, and intermixed with volcanic sand and pumice. The 
tusk of a mammoth has been procured from this formation, 
filled in the interior with solid travertin, wherein sparkling 
crystals of augite are interspersed, so that the bone has all the 
appearance of having been extracted from a hard crystalline 
rock *. 
These Roman tufas and travertins repose partly on marine 
tertiary strata, belonging, perhaps, to the older Pliocene era, 
and partly on volcanic tuff of a still later date. They must 
have been formed in small lakes and marshes, which existed 
before the excavation of the valleys which divide the seven hills 
of Rome, and they must originally have occupied .the lowest 
hollows of the country, whereas now we find them placed upon 
the summit of hills about 200 feet above the alluvial plain of 
the Tiber. We know that this river has flowed nearly in its 
present channel ever since the building of Rome, and scarcely 
any changes in the geographical features of the country have 
taken place since that era. 
When the marine tertiary strata of this district were formed, 
those of Monte Mario for example, the Mediterranean was 
already inhabited by a large proportion of the existing species 
* This fossil was shown me by Signor Riccioli at Rome. 
