Correlation of the North American with the British Lower Ordovician is a source of conflicting opinions resulting from different interpretations of the first appearance of graptolites with biserial scandent rhabdosome form in each area. The bi-serial scandent graptolites are considered to have originated and become well established in the British Faunal Province before they invaded the North American Faunal Province when a zoogeographic barrier was breached. Most of the North American Canadian Series (the Tetragraptus approximatus through Didymograptus protobifidus zones) is correlated with most of the British Arenig (the Didymograptus extensus and most of the Didymograptus hirundo zones). The North American Whiterock, Marmor (Chazy), and Ashby Stages (the Didymograptus bifidus through Glyptograptus cf. G. teretiusculus zones) are correlated with the highest part of the Arenig and the Llanvirn and Llandeilo Series in Britain (the upper part of the D. hirundo through the G. teretiusculus zones).