318 ORTMANN — DISTRIBUTION OF DECAPODS [Aprils, 



the Upper Jurassic (Neumayr, 1890, p. 329), which reach the 

 Pacific Ocean. Beds of the same age are known on the Aleutian 

 Islands and in Alaska. These deposits exhibit a peculiar character, 

 which has been called the boreal or arctic type, and in this respect 

 the Jurassic beds of the western coast of North America are very 

 important, since they agree with the boreal type. Neumayr con- 

 cluded from this that the Upper Jurassic Polar Sea sent an exten- 

 sion southward along the western coast of North America into the 

 North Pacific, and its fauna also extends in this direction ; by this 

 extension of the Polar Sea, east Asia was separated from North 

 America. Consequently there was no land bridge. 1 



These conditions of Upper Jurassic times continued, according 

 to Neumayr, into the Lower Cretaceous ; the Wolga-stage, with its 

 characteristic Aucella-beds, belongs in part to the Lower Cretace- 

 ous, and the Polar basin was also in the beginning of the Creta- 

 ceous in open communication with the northern Pacific. This is 

 represented in Koken's map (1893, pi- *)> although Asia and 

 North America approach each other considerably. This same 

 map, however, expresses, for the Upper Cretaceous, a separation of 

 the Polar Sea from the Pacific, and this land connection between 

 Asia and North America is preserved in Koken's map for the older 

 Tertiary (/. c, pi. 2). The evidence for this disconnection of the 

 oceans in the Upper Cretaceous time is given by Neumayr (1890, 

 p. 389-391) ; palaeontologically, we can trace a continuous Upper 

 Cretaceous ocean, including the northern Pacific from California 

 to Japan, which was connected with south India. This province 

 differs strikingly from the American-European (Atlantic) province ; 

 the Polar Sea was much reduced in size, and, to all appearances, 

 Siberia was largely dry land and was connected with North 

 America. 



Thus there is some evidence of the existence of a land connec- 

 tion between Siberia and Alaska, beginning at about the middle of 

 the Cretaceous period, and continuing up to the end of the Ter- 

 tiary. Whether this connection was continuous in time, or inter- 

 rupted at certain periods, is hard to decide ; at all events, it was 

 of such a character that an easy and free communication was pos- 

 sible between the respective parts, and this is expressed very dis- 

 tinctly in the faunas of the northern land masses, although the 



1 This Jurassic ocean forms apparently the continuation of the old Triassic 

 basin, comprising the Pacific and Arctic Oceans (see Neumayr, p. 266). 



