298 Royal Society : — 



Emmons, Hall, Logan, and Billings *, and is the fruit of their un- 

 wearied study in the city, and of their toil in the field. 



Recurrence. 



A few words on the subject of recurrence, or the vertical range of 

 Silurian life. 



What can be more unexpected, or more wonderful, than the up- 

 ward passage by a filial succession, through stages and epochs, of a 

 mollusk during centuries inconceivably numerous ! What an almost 

 interminable series of posterities must have followed the first ances- 

 tor! The doctrine of limited duration in species must have its 

 exceptions. 



The * Thesaurus ' enumerates 803 recurrents, or 1 2 per cent, of 

 the whole known life of the epoch — a very notable proportion, — 

 still leaving 6200 species faithful to one horizon. 



The synoptical Table E, compiled from the ' Thesaurus,' exhibits 

 many details, and may be trusted approximately, although about 400 

 species have been passed by for want of sufficient information. It 

 numbers separately the species typical of one horizon, and the species 

 frequenting more than one horizon (being recurrent). It also intro- 

 duces some of the greater genera, such as Orthis, Mtirchisonia, &c. 



The species are arranged under the heads " Primordial," " Lower," 

 " Middle," and *' Upper Silurian ;" and in the case of the recurrents 

 the number of horizons occupied by them is shown by the figures 

 2, 3, 4, 5. Thus we find that 69 Lower Silurian Trilobites occupy 

 two horizons, 1 5 three, and 2 five horizons. 



The percentage is stated in the last column, next to that containing 

 the total recurrence of each Order. 



The Primordial stage only gives 2*7 per cent, of recurrency. 



The Lower Silurian 16 „ 



The Middle 20 



The Upper 8 



The Orders vary greatly in respect to recurrency. There is none 

 among fossil fish. In Cystidea it is only 3 per cent., in Gomphoceras 

 5 per cent., and is greatest in Strophomena, 31 per cent. 



Although a considerable number of inferences have been pre- 

 pared, I shall only venture now to introduce a few. 



1 . Recurrence is universal, both as to time and place. 



2. Recurrences seem to be most numerous in the lower stages of 

 the epoch ; but further research may teach otherwise. 



3. Species do not often change their horizon, not even when 

 placed in countries far apart. 



4. The same species may be typical of a single horizon in one 

 country and recurrent in another. 



5. Recurrency shows that a mollusk is not necessarily confined 



* It is well to note that, under Sir William E. Logan's able superintendence, 

 we owe the splendid Primordial harvest gathered in Newfoundland and Anti- 

 costi to the diligence and skill of Mr. Richardson, an explorer in the employ 

 of the Canadian Geological Commission. 



