ART. 21 ORDOVICIAN TRILOBITES ULRICH 87 



the middle third. The Ashgillian is succeeded by the Skelgill beds. 

 These comprise a number of thin zones with species of Mofiograptus 

 and at the base a thin limestone with Atrypa flexuosa and directly 

 over this a black mudstone with Dimovphograptus. 



Just why the first appearance of monograptids, unheralded as it 

 usually is by a well-marked physical break, should determine the be- 

 ginning of the Silurian system and the close of the Ordovician is not 

 clear to me. It is merely an event in the course of Silurian history 

 and one that can hardly be expected to have been manifested at pre- 

 cisely the same time everywhere. It is no more important than the 

 first appearance of Fenestella in the Richmond or of Heirdtrypa in 

 the Brassfield or of Coelospira and Spirifer in the Clinton or of the 

 subsequent first appearances of many other generic types that became 

 abundant and lasted for long periods thereafter. 



In America we also find it troublesome to detect a satisfactory 

 physical or faunal boundary between the Richmond and the Alex- 

 andrian, and considerable difference of opinion as to the precise loca- 

 tion of the Medinan-Clinton boundary is notable in American litera- 

 ture. However, as regards the systemic boundary, the best informed 

 American stratigraphers — at least those who have learned their stra- 

 tigraphy from field observations in many areas rather than from lab- 

 oratory studies and comparisons of collections of fossils — are well 

 satisfied to follow the footsteps of the geologists of the first geological 

 survey of New York, who in the forties of the last century drew the 

 boundary between their Ontario and Champlain divisions of the New 

 York system at the generally clearly marked base of the Lower 

 Medinan. The official survey of New York has never, so far as I 

 know, receded from its position on this question except by substituting 

 British terms for New York names. 



After four brief but well-filled periods of field studies in Britain, 

 Scandinavia, and Bohemia my conviction that diastrophically well- 

 marked systemic boundaries essentially corresponding in age to those 

 worked out in America may also be determined on the east side of the 

 Atlantic is more firmly fixed than it was on my first visit to Europe 

 in 1922. 



Norway aoid Sweden. — This column requires little explanation. 

 Etage 5 of the Norwegian section and the Leptaena limestone of 

 Sweden are placed into the Silurian for practically the same reasons 

 as those that seemed to demand the removal of the British Keisley 

 and Drummuck formations from the Ordovician to the naturally de- 

 limited Silurian system advocated hj me. The proper placing of 

 the Norwegian Etage 4 and the Swedish Trinucleus {Tretaspis) 

 and Chasmops zones I find much more difficult. Regarding these 

 Scandinavian zones the Trinucleus zone seems at present to belong 



