274 



Mr. C. Lapworth on the Geological 



containing beds. In the second place we shall look upon them 

 from the zoological or palasontological side, noting the geolo- 

 gical date of the advent, culmination, and gradual extinction of 

 the known families, genera, and species, and fixing approxi- 

 mately their individual range in Palaeozoic time. 



Upper Cambrian RocJcs. 



Table II. Showing Range of Upper Cambrian 

 Rhabdophora. 



The oldest Rhabdophora hitherto detected are probably the 

 forms discovered by Mr. Linnarsson in the Olenus-beds of 

 Westrogothia, Sweden. These strata are possibly of a some- 

 what higher antiquity than the Shineton shales of Shrop- 

 shire, in which Dr. Callaway has discovered corresponding 

 forms. 



The only family of Rhabdophora as yet represented in these 

 Upper Cambrian graptolitiferous rocks is that of the Dicho- 

 graptidse, to which the vast majority of the forms belong 

 which mark the succeeding Arenig period. Indeed this frag- 

 mentary Cambrian graptolitic fauna appears at first sight to be 

 essentially of an Arenig type. One prime distinction, however, 

 can hardly be overlooked. In the Arenig formations the 

 more simple forms, such as Didymograptus and Tetragraptus, 

 greatly preponderate, the highly complex forms like Dicho- 

 graptus and Clonograptus being comparatively rare. In this 

 small Upper Cambrian assemblage, on the other hand, the 

 only forms recognized belong to some of the most highly 

 complex genera known — the British, Norwegian, and Swedish 

 faunas agreeing precisely in this respect. 



