40 



BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



The foremost American students of paleontology and paleogeog- 

 rapliy have recognized three major faunal realms in the Northern 

 Hemisphere, each of which was a center of development and dispersal. 

 These three realms are, first, the Atlantic; second, the Arctic; and, 

 third, the Pacific. Wlienever opportunity offered, the faunas of one 

 or more of these oceanic basins invaded the submerged portions of 

 the continent. Most frequently but a single fauna occupied the 

 interior of the continent; occasionally faunas from two of the major 

 regions of dispersal were present, and rarely all three may have been 

 represented contemporaneously. For North America the South 

 Atlantic center of dispersal was probably the most important, at 



■ — . 2H— ~~^fc:^"^ ^//^^. 



_c\v_-- — '^K' 





,„ . j^j / r^~^ 



^ =?T-*4 — 





A 



f^ V-^^.J^-r> 



r^^^r^^W\^/fi -^>^"~^ 



~;?r=^^^ 



^^« 





\ \ \\ \ ^sl/ 



"^--~_'^ /u 



31^t" 



\i^/^^^^)^^ 



Xv\^ 





^ 



'^l^^^^^^l 



\\\ \ 



' /C *^'- ^^>A^ 



^ 



V^ y^K^'i-n) \ 



» \ * \ 



\/^ Nv ^^V a \ 



^^ 





■^ Vj'^^v \ 



\ ^^ '•■■ ^ > 



/M^ 



C\ y Wr*'y ^•■^'■/S^^J ^/ 





\ 4^\*|^ 



■* ^^^b 



"w*-nX^ jyj^#R*m r^W— » 





\ \ ^*//*' v^ 



N^^r 





"^J,-'' \ .f^ 





/\ wfp 



\a )v 



^ 



y \ -^ / f \v ] y 'sW^ / ^j 





\ ^^ ' ^ 





\ /J \j^,V / %s *• , 



i \ ^\ 







e/- /J -'^^° % i 





" ■p'-iA <>L- L- \ 



& 



■•4w m-^h 



'^ — 1 — ' \ 





1?/ 



X'' / ! / / •^ 





XX 







\ Ky< 



^.~wa5\ 







\ 



S-'^^ml 







^ 



' // > ?«. /^ ^-T — ■ — /-— -^ 



j^ \ 



\ \ ^ 





S^-s^ o-sy ^&^ / 







^ 



y/ ^"^^ / / 







\V- 



y^jc/ ^-^ «/ Scl w 



^A ... 



lA ^^ 



«\ l^ 



Fig. 2.— Map of the Noeth Polar region, showing distribution of Silurian strata in North 

 America and northern Europe, "with the hypothetical shore lines. (After Wellee.) 



least in earlier and middle Paleozoic times, with the Arctic center 

 second in importance. The Ordovician and Silurian faunas of the 

 Baltic region and the central and northern parts of North America 

 have so much in common that it seems probable they were all derived 

 from the Arctic region, from which, whenever the opportunity 

 afforded, they migrated into the adjacent continental areas. The 

 identity of the middle Silurian faunas of North America and northern 

 Europe has been established by Weller (fig. 2), but the earlier Silu- 

 rian and Ordovician faunas have not hitherto been believed to show 

 any similar community. This belief may rest on the fact that the 



