OLEXELLUS AND OTHER GENERA OF MESONACID.i; 253 



and the closely related Pcrdciimias traiisitans range from Alabama 

 to Lake Champlain and down the St. Lawrence valley to the south- 

 eastern end of Labrador in the Atlantic Province. On the eastern 

 side of the Atlantic Olenelhis lapzvorthi is abundant in northwest 

 Scotland on Loch Maree. 



Olencllus has a wide distribution, and it may in the future be 

 found in Siberia and far to the north within the Arctic Circle on 

 both North America and Asia and adjacent islands. 



Hohnia has both an extended geographic and stratigraphic range, 

 especially if we consider with it the closely related Callavia and 

 IVaimcria. Hohnia rozvci in the lower portion of the known Lower 

 Cambrian horizon of Nevada is unknown elsewhere, and H. kjcrulfi 

 is limited to the Scandinavian area, but probably will be found to 

 extend eastward into Russia and possibly Siberia. Callavia is essen- 

 tially an Atlantic Province genus as the one species from Nevada, C. 

 F ncvadcnsis, is a more or less doubtful reference. 



ElJiptoccphala and Nevadia are each limited to a single species 

 and a narrow distribution and stratigraphic range. Mesonacis ver- 

 nio)itaiia occurs in western A'ermont on Lake Champlain, and it will 

 probably be found in the St. Lawrence River area. Mesonacis ? 

 inickzvitzi is probably a Mcsojiacis, but this awaits further proof. 



Nothing is known of the ^Mesonacidse on the Asiatic continent, and 

 the evidence for the presence of any of its forms in Australia or 

 elsewhere than as described in this paper is not sufficiently con- 

 clusive to justify my accepting it. I am prepared to learn that un- 

 doubted specimens have been found in Siberia and Australia, and 

 possibly Sardinia and to the north in Spain and France. 



With our present information, the Mesonacidas is confined to 

 western Europe and North America. The immediate descendants 

 of the family are probably Paradoxides about the Atlantic Basin, and 

 Redlichia [Walcott, 1905, p. 25] in eastern Asia, northern India, 

 and Australia. 



TRANSITION FRO^I THE MESONACID.^ TO THE 

 PARADOXIN^ 



The question of the transition from the Lower Cambrian fauna to 

 the ^Middle Cambrian fauna is one that has not been fully worked 

 out. That all of the genera of the Alesonacidse should disappear be- 

 fore the undoubted appearance of Paradoxides is a very significant 

 fact and to me indicates that there was a transition fauna in the 

 Atlantic Province, and that in most instances owing to shifting shore 



