CLASS I PELECYPODA 503 



arc initiated. The cliaracter of the Cretaceous is strongly influenced ]jy tlie aberrant 

 and sliort-livcd Chamacea and Rudistids. The Mutelidae, Pandoridae, Clavagellidae, 

 Poromyacidae, Crassatellitidae, Cryptodontidae, Petricolidae, true Solens, the Mac- 

 tridae and Saxicavidae, also take their origin during this period. 



With the beginning of the Tertiary a gradual approxiniation to present conditions 

 takes place. The Rudistae have disappeared, the Dysodonts are on the decline, and 

 the Teleodesniacean types on the increase. Nunierous Anatinacea, Leptonacea, Tridac- 

 nidae, Callocardiidae, Senielidae, Mesodesmatidae and Myacidae appear. At the close 

 of the Eocene, the wide distribution of many types now characteristic of warm- 

 ternperate, or tropical waters begins to be restricted ; and during the Miocene the 

 faunal boundaries of mollusks depending* upon temperature conditions are laid down 

 nearly on existing lines. 



The following table indicates more ejcactly the geological ränge of the families of 

 Pelecypods according to oiir present information : — 



[Tabl: 



