398 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



dawn of life ; then some Gasteropoda, a few Cephalopoda, and a few 

 Brachiopoda in the third group from below (Chazy). But in the 

 fifth group from below (Trenton), multitudes of Zoophyta, Bryozoa, 

 Brachiopoda (save Spiriferi), Orthocerata, and Trilobites spring forth; 

 but not a Lamellibranchiate. As species, they nearly all perish 

 with the advent of a new deposit ; but, as genera, they appear one 

 after another through the successive epochal centres, becoming mul- 

 tiplied in numbers and perfect in form. Then they lessen in numbers, 

 dwindle in size, and finally disappear. 



30. There is a close similarity in New York and Wales in the 

 increment and decrement of Zoophyta, Bryozoa, Echinodermata, 

 Brachiopoda, &c. ; that is, these fossils are numerous and few at 

 the same points of the Silurian scale. 



31. The same genera, species, and amount of individuals abound 

 or are few in the countries just named. Brachiopoda, Crustacea, 

 Orthocerata, are many ; Lamellibranchiates few. The extraordinary 

 opulence in fossils of the Rhenish Devonian strata does not obtain in 

 New York. In New York, however, according to our present list, 

 the Lower Silurian stage is the most fossiliferous ; in Wales, it is 

 the Upper. Future discoveries may change this condition of things. 



32. A remarkable feature in the uppermost four groups of New 

 York Siluria (the Lower Helderberg) is the substitution in them 

 of limestone for the arenaceous mud of the Welsh Ludlows, their 

 contemporaries. It has given them a Wenlock character. But it 

 is to be remembered that the Ludlow and Wenlock groups of Wales 

 are in close fossil-connexion, — 74 out of 31 1 species of organic remains 

 being common to both, or very nearly one quarter. 



I shall 'not proceed at present with these inferences into the 

 American Devonian system, although there is no want of interest. 

 I may just remark that many Silurian Brachiopoda and some other 

 molluscs work themselves up into the Devonian as representatives of 

 a common period. They may even be found in the Carboniferous 

 system, as has been proved by D'Archiac and De Verneuil to be not 

 uncommonly the case in Europe. 



The great ruling zoological principles of the Silurian system are 

 continued into the Devonian ; but in the latter we have the intro- 

 duction of Vertebrates in profuse variety, and of new and complex 

 types of Invertebrates in unwonted abundance, the old forms dying 

 out. 



