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SummaryFission-track dating of zircons and apatites from tuffs and bentonites has produced the first isotopic ages for the type sections of the Ordovician and Silurian Systems. In the Ordovician the following ages have been determined: lower Arenig 493 Ma, lower Llanvirn 487 Ma, lower Llandeilo 477 Ma, upper Caradoc 463 Ma and upper Ashgill 434 Ma. In the Silurian, the following: lower Llandovery 437 Ma, lower Wenlock 422 Ma, upper Wenlock 414 Ma and Ludlow 407 Ma. The Ordovician-Silurian boundary is interpreted as occurring at about 436 Ma. Three North American Rocklandian bentonites yielded zircons whose ages average 453 Ma. This is about 10 Ma younger than supposedly correlative units in the British type sections.

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Di8cussion.-'I'his species is very similar to Orthambonites .swanensis :from which it differs by its smaller size and by lack o:f multiple striae between costae. Previously I have referred this species to 0. c:f. 0. swanensis (Ross, 1964b, p. C18, C19, C68). 0. minusculus Phleger possesses a more auriculate outline and more angular costae and inters paces. Although 0. paucicostatu8 has a similar outline and about the same number o:f costae, it is a much larger species with only two to three costae in a space o:f 5 mm at the :front o:f the shell. 0. dinorthoides is a large :form like 0. pauaicostatus in outline and costal spacing, but it has costae and interspaces o:f about equal width.