382 



Dr. J. J. Bigsby's Brief Account of the [Feb. 21, 



dividuals, nevertheless, being prodigiously numerous. Trilobites are here 

 very few. 



Potsdam Sandstone is rich 'in Trilobites, Brachiopoda, and Fucoids, but 

 in every other form is very poor ; and yet it possesses a Cystid. 



In the Primordial group, therefore, we find numerous representatives of 

 nearly every marine Invertebrate ; and we have a startling example of the 

 sudden development in very early times of the highest types of moUuscan 

 life, Nautili, Lituites, Trilobites, Protichnites, &c., dwelling, even then, in 

 well adjusted communities. 



Most of these facts are taken from the ' Thesaurus but this interesting 

 portion of the * Thesaurus ' itself is the gift of Barrande, Emmons, Hall, 

 Logan, and Bilhngs *, and is the fruit of their unwearied 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 Silu- 

 rian life. 



What can be more unexpected, or more wonderful, than the upward 

 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 ancestor ! The doctrine 

 of limited duration in species must have its exceptions. 



The 'Thesaurus' enumerates 803 recurrents, or 12 percent, 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 maybe trusted approximately, although about 400 species 

 have been passed by for want of sufficient information. It numbers sepa - 

 rately the species typical of one horizon, and the species frequenting more 

 than one horizon (being recurrent). It also introduces some of the greater 

 genera, such as Orthis, Murchisonia, &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, 15 

 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 



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

 tlie splendid Primordial harvest gathered in IS'ewfoundland and Anticosti, to the dili- 

 gence and skill of Mr. Richardson, an explorer in the employ of the Canadian Geo- 

 logical Commission. 



