X PREFACE. 



of organic change in this direction. While discussing certain generic and 

 specific forms as characterizing known geological horizons or certain groups of 

 strata, we had not yet taken into consideration the fact that modifications of 

 organic types had been coincident with every change or progress in geological 

 time. The great law of progress through long intervals had been everywhere 

 recognized in geological science, but just how or in what manner these changes 

 liad supervened had rarely been shown in detail. Certain fossil genera have 

 Ion"- since been recognized as Silurian, others as Devonian, and others as 

 Carboniferous, but these are never entirely restricted to the formations which 

 they are said to characterize. They have all doubtless been derived from 

 some remote progenitor, and at certain horizons, or throughout certain forma- 

 tions have become so abundant and so fully developed, that they are said to 

 characterize that stage or formation. The most abundant and extravagant 

 forms among fossil organisms can usually be traced to some parent stock of 

 more modest pretensions, and in their early appearance, represented by few 

 individuals. 



As stated, the studies of the Brachiopoda to the close of Volume IV of the 

 Palaeontology had shown the importance of some investigation which should 

 deal directly with these questions. And moreover the science demanded the 

 results of such an investigation in aid of its future progress. 



The original conception and plan of the work which the author had 

 proposed to himself was a very simple one, viz.: to select the earliest 

 representative of a genus in any of the geological formations and to follow it 

 through all its manifestations and modifications in geological time, to its final 

 disappearance ; or so far as these modifications should appear in the Palaeozoic 

 rocks, to which he had limited his research.* With the knowledge then 

 possessed and with the collections at his disposal he had supposed that the 

 result of such an investigation could be embraced in a supplementary part to 

 Volume IV, and under this title the work was announced. This study was 

 commenced very soon after the publication of that volume and its general plan 



*The (lirticnlty of iirocuiin-,' .«iifficifintly abuiifliiiit and characloi-istic collections of the latei- forma- 

 tions was in itself a sufficient baiTiei-, and the -scope of the woik did not contemplate the discussion of 

 Mesozoic and later genera, except in an incidental manner. 



