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.]