164 F. R. Coicper Reed — Evolution of Cheirurus. 



consecutive series of transitional forms can be traced upwards from 

 it to Crotaloccplialus. , 



The stratigraphical gap between Etages C and D in that area may 

 account for the absence of the Anacheirurus and early Eccoptocheile 

 forms. 



In the northern province, on the other hand, the zoological hiatus 

 occurs between the early Eccoptocheile and its nearest ally, Nieszkow- 

 skia, on the one side, and Cyrtometopus on the other side. The 

 Bohemian Ch. claviger group supply the missing links, and we may 

 not unreasonably suppose that there was some communication, if 

 only occasional and intermittent, between the two provinces. The 

 manner in which the Cyrtometopus branch is connected with the 

 earlier forms of the whole genus seems to indicate this. It does not 

 appear improbable that a few migrants of the Ch. claviger group 

 wandered north from Bohemia, and, meeting with changed conditions 

 and a new environment, developed rapidly into the peculiar northern 

 species of Cyrtometopus, and branched off into the Nieszkowskia, 

 Sphmrocoryphe, and Pseudosphcerexochus types ; while their southern 

 contemporaries and congeners, under less variable and more uniformly 

 changing conditions, pursued the steady, undeviating march of direct 

 development without giving off side-branches. 



The physical or biological conditions of northern Europe, moreover, 

 appear to have affected the pygidia in a special way, leading to the 

 formation of the Cyrtometopian type, which is common to the majority 

 of the northern species of the subgenus Cheirurus ; while in Bohemia 

 the same subgenus has pygidia closely resembling those of the 

 ancestral Eccoptocheile. An intercommunication of the two provinces 

 is suggested by the presence of some southern types in the north, 

 especially in Great Britain, where the southern type of pygidium in 

 the subgenus Cheirurus is predominant. 



The subgenus Crotalocephalus, with its uniform characters 

 throughout North and South Europe, appears to have had its birth- 

 place in the southern province in Silurian times, and to be a direct 

 offspring of the typical southern forms of Cheirurus (sens. str.). In 

 the Devonian period it overspread the northern area as well, but did 

 not suffer any further modification in spite of this enlargement of 

 its area of distribution. 



LIST OF PRINCIPAL EUROPEAN SPECIES OF THE GENUS 

 CHEIRURUS. 



Cheirurus {Anacheirurus, nom. prop.) Frederici (Salter). British only: Upper 



Tremadoc, North "Wales. 

 Cheirurus {Eccoptocheile) Sedgwicki (Salter) . British only : Llandeilo Flags, South 



Wales. 

 ) claviger (Beyrich). 

 ) Guillieri (De Tromelin). 

 ) pater (Barrande). 

 ) marianus (De Verneuil). 

 ) perlata (Hawle and Corda). 

 ) curta (Hawle and Corda). 

 ( j, ) Burocheri (Ronault). 



