STRATIGRAPHIC CLASSIFICATION PALEONTOLOGIC CRITERIA 503 



the close of the Paleozoic, however, the Arctic brachiopods became as 

 much or more like the Pacific types. 



But few of the described early and middle Paleozoic pelecypods belong 

 to the Pacific realm. By far the most of these shells originated in the 

 middle Atlantic from which they spread into the continental seas of 

 southeast North America and Europe. In later Paleozoic ages, however, 

 a considerable development of the class occurred in the Pacific faunas. 

 Some Cambrian fossils have been referred to the Pelecypoda, but all of 

 these that I have seen, and concerning which there is no question as to 

 their Cambrian age, have proved to be bivalved Crustacea. The oldest 

 unquestionable pelecypods known are found in the Saint Peter sandstone 

 in the Mississippi Valley. This suggests the Caribbean sea or the south 

 Atlantic as the probable center of origin of the class. The first gastropods 

 having invaded the North American continent from the same direction 

 long before suggests further that the pelecypods were derived from the 

 primitive bilaterally symmetrical "patelloid" types of that class. 



The oldest, in everywise unquestionable, fossil record of the coiled 

 gastropods^* and of cephalopods is found in the Ozarkian rocks. In 

 fact these classes constitute the most important parts of the middle and 

 later faunas of this period as developed in the Mississippi Valley. As 

 but few species of these classes are found elsewhere in rocks of this age, 

 it is assumed that the gastropods and cephalopods originated in oceanic 

 basins to the south of the Mississippi embayment, that is in the Gulf of 

 Mexico, Caribbean Sea or South Atlantic. From there they spread to the 

 north and west, attaining before the close of the Canadian period rather 

 general distribution in the continental seas of North America. However, 

 judging from the Baltic section, they seem not to have reached the 

 European side of the Arctic until well into the Ordovician. About the 

 same time, or perhaps in the somewhat later Black River epoch, certain 

 types of cephalopods, like Gonioceras and Actinoceras, spread from the 

 Arctic into the Pacific. The development of the Eopaleozoic gastropods 

 and cephalopods, also the pelecypods, in the Bohemian-Mediterranean 

 province, seems to have been largely independent. Possibly they were 

 derived from Pacific ancestors, but the major part more likely originated 

 in preceding south Atlantic connections. In the Neopaleozoic and Penn- 

 sylvanian ages both the gastropods and the cephalopods developed along 

 similar lines in the three provinces of the Atlantic realm. Their Arctic 



**A few small, unsymmetrlcally involute shells, that resemble Platyceras and Cyrto- 

 lites and which may possibly be true Gastropoda, occur in the Lower Cambrian in Nova 

 Scotia and Newfoundland. The same type of shell extends up into the Ozarkian. A 

 species of the latter was described by Walcott as Platyceras minutissimum. 



