﻿Vol. 6 1.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxXV 



As the physical conditions of the deeper parts of the ocean are 

 more uniform over wide areas than those of the shallows, the deep- 

 water benthoal forms are more valuable for purposes of correlation 

 of remote sediments ; and this is, of course, especially true of the 

 forms belonging to the vagrant benthos. 



But it is to the members of the nekton and the plankton (in- 

 cluding in the latter the mero-plankton and the pseudo-plankton) 

 that we turn with the greatest confidence as being likely to furnish 

 us with the means of correlating strata over wide areas. These 

 easily-distributed organisms may have spread over wide tracts of 

 ocean soon after they came into existence, and if their period of 

 eudurance be long as compared with the time taken for spreading 

 from their centres of origin, they will give most important in- 

 formation for purposes of correlation. As their remains may be 

 embedded in the sediments of the shallows and the depths alike, 

 they are also useful for comparing the beds of shallow-water with 

 those of deep-water origin ; whereas the benthoal forms of these 

 deposits will probably differ. To take an example, we may allude 

 to the graptolites, which, according to Prof. Lapworth, constituted 

 members of the pseudo-plankton of Lower Palaeozoic times. The 

 faunas of the normal graptolitic type of sediment often consist 

 almost entirely of graptolites ; while the deposits of the shallows of 

 the same ages usually yield a variety of organisms belonging to 

 many different zoological groups. Here and there, however, we 

 come across a few graptolites associated with the other fossils, 

 which enable us to correlate the shallow -water deposits with those 

 of the normal graptolitic type, as has been done so successfully by 

 Prof. Lapworth himself in the case of the richly-graptolitic deposits 

 of the Moffat district and the contemporaneous shallow-water 

 sediments of the Girvan area. 



In future work on the faunas of the sediments, the effects of 

 homoeomorphy must be regarded more carefully than has been done in 

 the past. Cases of contemporaneous homoeomorphy, such as appear 

 frequently to occur among the graptolites, do not vitiate the conclu- 

 sions of the stratigrapher ; but, when homoeomorphy occurs in the 

 case of organisms which lived at different times, it may lead the 

 student to arrive at wrong conclusions. For instance, the graptolites 

 of the genus Azygograptus might at one time have been mistaken 

 for forms of Monograptus ; indeed, before the significance of the 

 position of the sicula was appreciated, the diagnosis of the genus 

 Monograptus was so drawn up that forms of Azygograptus would 

 have been referable to that genus. The apparent similarity in 



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