272 



PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



the minority, being only 121 in 903, or ^ths, for New York, and 

 196 in 797, or nearly ^th, for Wales and Siluria. In by far the ma- 

 jority of cases the vertical range is short, and confined to one act 

 of reappearance, as may be seen in Table XI., in which the upper 

 line represents the number of groups or sections inhabited by the 

 fossils enumerated in the lower lines. Thus, in ISTew York 93 fossils 

 recur once, 16 twice, and so on. 



In the above Table, the numerical differences, as regards the 

 country, are more apparent than real. They much depend on the 

 smaller subdivisions employed in New York, — 19 in that State, 

 and only 7 in Wales. Eecurrence in most instances taking place 

 in contiguous groups, the same species is apt to be counted twice, 

 from the occasional difficulty of determining the true terminal 

 planes of each group. American authors unite with those of all 

 other countries in testifying to the great mortality occurring at the 

 end of the successive epochs, whether in Europe, America, or else- 

 where. The useful Table XII. places this fact in a striking point 

 of view for the Silurian system of New York. It is intended to 

 show, with some exactness, that the specimens common to the 

 various epochs or sections are usually few in comparison with the 

 whole fauna of the period, and that even the swarming hfe of 

 Trenton Limestone and Niagara rocks mostly perished, never to 

 return. 



Many useful observations may be made on this Table ; but as 

 most of them are obvious and others will be made afterwards, we 

 shall here abstain, excepting on two points. Utica Slate, which 

 has but few fossils of its own, contains an unusual number of such 



Table XIII. — Exhibiting the Typical and Recurrent Fossils of 

 Wales, with their Ratios. 



Fossils. 



IS 

 o 



s 



6 



Fossils. 



1 



1 



o 



6 



i 



PlantsB 



12 



1 



56 



39 



76 



23 



131 



107 



1 







28 



10 



1 



5 



32 



59 



t 



1 



4 



2 



1 1 



Monomyaria. . . 



Dimyaria 



Gasteropoda . . . 

 Heteropoda ... 

 Pteropoda . . . 

 Cephalopoda 

 Pisces 



15 

 53 

 40 



8 



7 



37 



9 



7 

 13 

 14 



5 



3 

 15 



9, 



i 



i 



1 



3r 



3 



T 

 2 

 ?■ 

 2 



Amorphozoa 



Zoophyta 



Bryozoa 



Crinoidea 



Annelida 



Crustacea 



Brachiopoda 







"S 



as are found in other epochs, particularly in the two contiguous, 

 below and above. The Brachiopoda furnish the most numerous 

 links of connexion, because this order generally has great viability. 

 Further research will undoubtedly discover many more recurrent 

 fossils in the Lower Helderberg series. 



Table XIII. sets forth the ratio of recurrent fossils to typical in 



