﻿Yol. 6 1,] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxi 



of many forms are still undetermined. This will, of course, become 

 a less serious matter as our knowledge progresses. 



Another difficulty is due to discontinuous vertical distribution of 

 various organisms in many areas, causing certain organisms to 

 recur after their disappearance from an area for some time. Such 

 recurrences may take place after longer or shorter intervals of 

 time. Remarkable examples of recurrence after long periods are 

 furnished by the trilobite JEglina rediviva, which is found in the 

 beds D x , D 3 , and D. of Bohemia, but not in D 2 and D 4 , as was long- 

 ago pointed out by Barrande ; by the Olenids, which, abounding in 

 rocks of Lingula-Flsig age, are rarer in those of Tremadoc age, and, 

 after apparent absence in Arenig and Llandeilo rocks, appear in 

 some force in Caradoc times in the forms of the genera Gyplioniscus 

 and Triarthrus ; and by yet another trilobite, Ampyx, which, essen- 

 tially an Ordovician form, is known in the Llandovery rocks of 

 Scandinavia and Britain and in the Ludlow rocks of Britain and 

 Bohemia, but has not yet been found in strata of Wenlock age- 

 These forms must, of course, have existed somewhere during 

 the periods when the rocks were deposited in which the fossils 

 have not been found, and will no doubt be eventually discovered. 

 These recurrences after long periods of apparent absence would be 

 difficulties in the way of the correlation of strata by their organic 

 contents, if they occurred with frequency ; but they are by no means 

 common, and usually exist in the case of a small percentage only 

 of the total forms of the fauna. 



Recurrence on a minor scale is quite common, as we might 

 naturally expect, and even when the remains of an organism are- 

 distributed throughout a certain thickness of strata, they are apt 

 to occur in exceptional abundance at some horizons, and to be 

 sparsely distributed at others. Reference is made to this point, in 

 order to show the importance of obtaining suites of fossils from all 

 parts of a mass of sediment so as to become properly acquainted 

 with the characters of its fauna. The absence of a fossil from 

 beds in which it might be expected to occur is often asserted after 

 insufficient observation, the particular belt of rock in which the 

 remains of that fossil are preserved not having been carefully 

 examined. 



Again, the blurring of the tracings made by organisms is due to 

 the changes in faunas not having taken place suddenly, but by 

 extinction of one form at one time and of another later, and 

 similarly with their incoming. Owing to this, we cannot define a 



