CLASS I PELECYPODA 503 



are initiated. The character of the Cretaceous is strongly influenced by the aberrant 

 and short-lived 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 approximation to present conditions 

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

 the Teleodesniacean types on the increase. Numerous 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- 

 temperate, 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 exactly the geological range of the families of 

 Pelecypods according to our present information : — 



[Table 



