Geology . nce and Asia lay along thePacific border, while the migrating tract between the Dakota province and Asialay in the Mackenzie basin and along the Arctic border, the two provinces onlycoming into free communication far to the westward, it is not difficult to under- 1 Jura and Trias at Taylorville, California, Bull. Am. Geol. Soc. Am., Vol. Ill,p. 410. THE JURASSIC PERIOD. 93 stand how the Aucella, with favoring currents and temperatures, could migratefrom California into Russia without migrating into the Dakota province. Onthe other hand, species migrating from Russia might easily take
Geology . nce and Asia lay along thePacific border, while the migrating tract between the Dakota province and Asialay in the Mackenzie basin and along the Arctic border, the two provinces onlycoming into free communication far to the westward, it is not difficult to under- 1 Jura and Trias at Taylorville, California, Bull. Am. Geol. Soc. Am., Vol. Ill,p. 410. THE JURASSIC PERIOD. 93 stand how the Aucella, with favoring currents and temperatures, could migratefrom California into Russia without migrating into the Dakota province. Onthe other hand, species migrating from Russia might easily take either the Pacificroute to the Californian provinces, or the Arctic-Mackenzie route to the Dakotaprovince. If this were the geographical configuration, future research willprobably show that faunas originating on the Pacific coast in America had adistribution like the Aucella, and that faunas originating in the Dakota provincehad a distribution through the Arctic regions and westward into northern Russia,. Fig. 369.—Cephalopods: a, Cardiocercs cordiformis M. and II.: b, Ncumcyria henryiM and H ; c, Brtcmnites densus M, and H Pelecypods: d, Camptonectes belli-strintus Meek; e, Mytilus itfhitei Whitf.; /, Grammatodon inornatus M. and H ;g, Pseudomootis curtri (Hall); h, Ostrea strigilcrula White. Bkachiopods: i, Rhyn-chonella grtathophora Meek; j, Lingula brevirostra M. and II. 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 ruleIn rare cases, species from one American province might reach the other via thejunction of their migrating tracts in Asia, or wherever it may have been. Atthe time of maximum transgression of the sea, more direct communication betweenthe American provinces might naturally have been established. Presentknowledge of the Jurassic fauna of the Arctic islands is too scant to throw muchlight upon this matter. Ammonites macclintocki, closely related to
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