THE JURASSIC PERIOD. 



93 



stand how the Aucella, with favoring currents and temperatures, could migrate 

 from California into Russia without migrating into the Dakota province. On 

 the other hand, species migrating from Russia might easily take either the Pacific 

 route to the Californian provinces, or the Arctic-Mackenzie route to the Dakota 

 province. If this were the geographical configuration, future research will 

 probably show that faunas originating on the Pacific coast in America had a 

 distribution like the Aucella, and that faunas originating in the Dakota province 

 had a distribution through the Arctic regions and westward into northern Russia, 



Fig. 369. — Cephalopods: a, Cardioccrcs cordiformis M. and H.; b, Ncurnayria hcnryi 

 M and H ; c, Belemnites densus M. and H Pelecypods: d, Camptonectes belli- 

 striatus Meek; e, Mytilus itihitei Whitf.; /, Gramnmtodon inornatus M. and H ; 

 g, Pseudomo'wtis curta (Hall); h, Ostrea strigilecida White. Bkachiopods: i, Rhyn- 

 chonella gnathophora Meek; /, Lingula brevirostra M. and H. 



rather than into the California province, while Russian forms entered both prov- 

 inces, and the South Asian forms entered only the Californian province as a rule 

 In rare cases, species from one American province might reach the other via the 

 junction of their migrating tracts in Asia, or wherever it may have been. At 

 the time of maximum transgression of the sea, more direct communication between 

 the American provinces might naturally have been established. Present 

 knowledge of the Jurassic fauna of the Arctic islands is too scant to throw much 

 light upon this matter. Ammonites macclintocki, closely related to A. concavus, 

 has been found on Prince Patrick Island, and A. wosnessenski, A. biplex, Belem- 

 nites paxillosus, and Pleuromya unioides at Cook's Inlet. 1 



The Jurassic fauna of the Dakota province belongs to a late epoch of the 

 period, which implies perhaps that the Arctic sea did not extend its elongate 

 arm so far south until near the time of the great stage of sea transgression of 

 which it constituted one of the striking features. 



1 Dana's Manual, p. 760. 



