(fig. 219), most of the living forms of which are found in warm regions, inhabiting fresh or brackish waters, undergoes a vast development in the Eocene period, where it 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, together with the shells of true Land-snails.  Amongst these may be mentioned numerous species of Limnoea (fig. 220), Physa (fig. 221), Melania, Paludina, Planorbis, Helix, Bulimus, and Cyclostoma (fig. 222).

[Illustration:  Fig. 215.—­Turbinolia sulcata, viewed from one side, and from above.  Eocene.]

[Illustration:  Fig. 216.—­Cardita planicosta.  Middle Eocene.]

[Illustration:  Fig. 217.—­Typhis tubifer, a “siphonostomatous” Univalve.  Eocene.]

[Illustration:  Fig. 218.—­Cyproea elegans.  Eocene.]

[Illustration:  Fig. 219.—­Cerithium hexagonum.  Eocene.]

With regard to the Cephalopods, the chief point to be noticed is, that all the beautiful and complex forms which peculiarly characterised the Cretaceous period have here disappeared.  We no longer meet with a single example of the Turrilite, the Baculite, the Hamite, the Scaphite, or the Ammonite.  The only exception to this statement is the occurrence of one species 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 Ammonitidoe, as a family, had become extinct before the Eocene strata were deposited.  The ancient genus Nautilus still survives, the sole representative of the once mighty order of the Tetrabranchiate Cephalopods.  In the order of the Dibranchiates, 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 been found in any Tertiary stratum; but the internal skeletons of Cuttle-fishes (such as Belosepia) are not unknown.

[Illustration:  Fig. 220.—­Limnoea pyramidalis.  Eocene.]

[Illustration:  Fig. 221.—­Physa columnaris.  Eocene.]

[Illustration:  Fig. 222.—­Cyclostoma Arnoudii.  Eocene.]

Remains of Fishes are very abundant in strata of Eocene age, especially in certain localities.  The most famous depot for the fossil fishes of this period is the limestone of Monte Bolca, near Verona, which is interstratified with beds of volcanic ashes, the whole being referable to the Middle Eocene.  The fishes here seem to have been suddenly destroyed by a volcanic eruption, and are found in vast numbers.