scHucHEET.] GEOGRAPHIC DISTRIBUTION. 17 



the very rapid increase of tlie class during the Ordovician, its culmina- 

 tion in the Devonian era, and its rapid decline in the Carboniferous. 



Of the 230 established Paleozoic genera, not fewer than 18G are rep- 

 resented in Korth America. 



GEOGRAPHIC DISTRIBUTION. 



The geographic distribution of North American Paleozoic brach- 

 iopods is extensive, since 30 per cent, or 537 species, had great areal or 

 horizontal dispersion. One hundred and seventeen species are found 

 in both the Mississippian and Cordilleran seas, and of these 36 are 

 also known to occur in foreign countries. The number of species com- 

 mon to North America and other continents, however, is 121. 



When considered chronologically, it is observed that 20 per cent of 

 the Cambrian brachiopods have great geographic distribution, and that 

 this increases to 32 per cent in the Ordovician, Silurian, and Devonian, 

 and declines to 28 per cent in the Carboniferous. Greatest specific 

 dispersion, however, is most noticeable in the Devonian and Carbon- 

 iferous, where Atrypa reticularis, Leptiena rhomboidalis, Orthothetes 

 cremstriatus, Productus scmireticulatus, P. punctaius, RhyncJwneUa 

 plenrodon, Spirifer disjunctus, and S. striatus have almost world-wide 

 distribution and great vertical or chronologic range. Many similar 

 species common to America and several European countries could be 

 mentioned. 



Specific distribution increases with ordinal rank. In the radical order 

 Atremata 25 per cent had dispersion, increasing to 27 per cent in the 

 Neotremata, and to 32 per cent in the Protremata and Telotremata. 



From the above considerations it is evident that brachiopods, as a 

 rule, can not be of great value in correlating over wide areas minor 

 Devonian, but particularly Carboniferous, horizons. In the Cambrian, 

 Ordovician, and Silurian, however, these fossils are of great value for 

 strati graphic purposes. Since post-Paleozoic brachiopods are not com- 

 mon in America, they can have little stratigraphic value, but in the 

 Trias and Jura of Europe, where species and individuals are common, 

 reliance can be placed upon them, and they are there regarded as next 

 in importance to the Ammonoidea for correlation. When paleontology 

 shall have advanced sufficiently, so that extracontinental correlation 

 of Paleozoic formations can be taken up in detail, it Avill be seen that 

 brachiopods, because of their wide dispersion, abundance, and favora- 

 ble preservation, will be of great service in working out paths of 

 migration and intercommunicating oceanic basins. 

 Bull. 87 2 



