Rbuport of the State Geologist. 161 



. had shown the necessity of subdividing many of the older recog- 

 nized genera, which had become the receptacle for forms having 

 external siniilarit}^ to the typical members of the several groups, 

 but possessing quite dissimilar internal structure. The natural 

 disinclination to propose new generic terms for members of a 

 class of fossils which had been so widely and thoroughly studied 

 in Europe, operated as a restriction in the erection of new names. 

 However, it became necessary to describe in those volumes and 

 in cotemporary papers some thirty-one new generic forms and to 

 suggest the necessity for farther separation among the hetero- 

 genous assemblages. These studies, made with fairly good col- 

 lections, and ranging through the Silurian and Devonian faunas, 

 could not fail to attract attention to the different external aspects 

 and interior characters of forms known under the same generic 

 terms, and considered as distributed through all the Palaeozoic 

 formations. Although the genera thus far proposed had not 

 been based upon a recognition of their appearance and duration 

 in geological time, yet the student could not fail to discover evi- 

 dences of organic change in this direction. While discussing 

 certain generic and specific forms as characterizing known geo- 

 logical 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 had supervened had rarely been 

 shown in detail. Certain fossil genera have long since been 

 recognized as Silurian, some as Devonian, and others as Carbo- 

 niferous, but these are never entirely restricted to the formations 

 which they are said to characterize. They have all doubtless 

 been derived from some remote progenitors, and at certain hori- 

 zons, or throughout certain formations have become so abundant 

 and so fulh^ 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 lY of the Palaeontology had shown the importance of 



