vol. 67.] the avonian of burrington combe. 389 



(6) Comparison of the Upper Paleozoic Corals with the Lower 

 Paleozoic Graptolites, in regard to Occurrence, Distri- 

 bution, and Evolution. (A. V.) 



(i) The coral and graptolite- bearing beds, respectively lime- 

 stones and mud stones, in their typical development build up rock- 

 masses of great thickness and lithological uniformity — the layers 

 that are actually fossiliferous forming but a small proportion of 

 the whole mass. Both coral-limestones and graptolite-shales can 

 alike occur intercalated in a series of typically arenaceous deposits ; 

 and, furthermore, both groups of fossils are occasionally found in 

 indisputably shallow-water deposits. 



(ii) Corals and graptolites share the disadvantage of being rare 

 or absent in certain types of deposit. 



(iii) Within each successive zone, certain genera of corals are of 

 world-wide distribution, and the time taken to bring about this 

 geographical expansion was, reckoned zonally, insignificant. 



Of such widespread genera we may notice : — Zaphrentis, LitJio- 

 strotioa. and the Clisiophyllids (both simple and compound). 



As examples of the zonal contemporaneity of faunal history we 

 may cite the following : — 



Lithostrotion (both massive and dendroid) enters in the early 

 Yisean in Xorth America, Britain, and Belgium. 



Lonsdalia characterizes the uppermost Yisean of Tian-Shan, 

 Belgium, and Britain. 



The coral indices occur in the same order wherever they are 

 known ; and, in the majority of cases, knowledge of the zonal series 

 in one area will unravel the succession in another. As examples 

 may be cited the coral sequences in Belgium, Northern France, the 

 South-Western Province, and the area described by Prof. Garwood. 



The truth of these statements when applied to graptolites has 

 long been recognized and utilized, since it was first pointed out in 

 the classical publications of Charles Lapworth. 



(iv) If, however, of two zonal stratigraphers, working inde- 

 pendently in two different areas, each drew up the series of zonal 

 indices best suited to his own area, the two series would probably 

 differ in two particulars, namely : — 



(1) The number of zones would differ according to the variability or 

 uniformity of the rocks and, in rough proportion to this factor, according 

 to the extent of the area studied. 



(2) The actual indices employed would differ specifically, although the 

 genera employed would usually be in accord. 



These facts are patent in the case of the graptolites, if we com- 

 pare the zones suggested for Bohemia, Scania, Lakeland, North 

 Wales, and South Wales. 



In the case of the corals it is sufficient to examine the time-scales 

 drawn up by Prof. Garwood and myself. 



(v) Evolution. — It is evident that, when the history of all the 

 dominant gentes of any fossil group has been definitely established, 

 the co-occurrence of known mutations of several of these gentes 



