298 



ME. E. S. COBBOLD ON TRILOBITES 



[Aug. 191 1, 



Helena Group. — The Olenellus Limestone. 



Microdiscus helena Walcott. 

 Micmacca (?) ellipsocephaloidesCohbold 



Do. 



do. 



Do. 



Do. 



do. 



Callavia callavei Lap worth. 



Callavia cartlandi Eaw MS. 



Ptychoparia (?) attleborensis S. & F. 



Ptychoparia (?) annio Cobbold. 



Iphidea (?) , Kutorgina (?) , Lingula (?) , 

 Linnarssonia (?), Hyolithus, Hyo- 

 lithellus, and Stenotheca, awaiting 

 critical determination. 



var. senior Cob- 

 bold, 

 var. strenuel- 

 loides Cobbold. 

 do. yai'.spinosa Cob- 

 bold. 

 Micmacca (?) parvula Cobbold. 



The generic references, particularly those in the lowest of the 

 three faunas, are more or less provisional, and will require careful 

 revision, before the divergences shown in the table between the 

 component groups of the Protolenus- Callavia Fauna are thoroughly 

 established. It may well be that further study of the forms, or 

 further work in the field, will bring them much closer together 

 than they seem to be at present. 



As they now stand, there is no species of trilobite which passes 

 the higher of the two dividing-lines, and even the genera seem 

 entirely different. The lower dividing-line, according to my present 

 determinations from very fragmentary specimens, is crossed by one 

 important species, Callavia callavei Lapw. It seems probable that 

 some of the brachiopoda, Hyolithidse, etc., will also be found to 

 pass the dividing lines, and so link up the three groups into one 

 fauna. 



The divergence between the Protolenus- Callavia Fauna and the 

 Groomii Fauna is very complete. No genus among the trilobites 1 

 passes from one to the other, and the forms of Stenotheca and 

 Hyoliiliellus that I have collected from the upper beds are quite 

 distinct from those of the lower. 



This divergence of faunas is accompanied by a well-marked 

 unconformity to which I refer in more detail below ; but I would 

 again call attention to the Black Limestone of Comley Quarry, 

 which comes in between the Grey Limestones with Microdiscus 

 lobatus and the Quarry-Ridge Grits. It contains a group of fossils 

 (trilobites, Hyolithidse, brachiopoda, etc.) that is, possibly, inter- 

 mediate between the Protolenus- Callavia and the Groomii F'aunas. 

 The trilobites are, however, so fragmentary and so difficult of ex- 

 traction, that I am compelled to await the evidence afforded by 

 the other fossils, before I can indicate the relations of this band to 

 the strata above and below it. 



The Unconformity. 



The evidence for the unconformity between the Middle and the 

 Lower Cambrian of Comley formerly consisted in the included blocks 

 of the conglomeratic base of the Quarry-Ridge Grits, and in the 



1 It must be noted that the genus Microdiscus reappears in the Davidis 

 Fauna, as in other regions. It is very doubtful whether the Ptychoparice (?) of 

 the Olenellus Limestone are properly referable to Corda's genus. 



