386 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



recognized are associated with the selected genotype, the resulting group 

 of species constituting the author's conception of the genus. Subsequently 

 other species found to possess the generic characters are added. In other 

 cases the species originally or subsequently associated into a single genus 

 may be found to embrace two or more groups of species sufficiently distinct 

 to be ranked as separate genera. In the revision the old name is retained 

 for the group containing the original genotype, other generic names, 

 either new or previously established, being used for the eliminated groups. 

 In stratigraphic classification a similar selection of a type — in this case 

 an especially characteristic or persistent bed or f aunal association, or some 

 definite period of time — would be of manifest advantage to subsequent 

 investigators. The practical application of such principles seems clear 

 enough to render examples unnecessary. Suffice it to say that progressive 

 reconstruction would be possible without totally wrecking previous nomen- 

 clature. And the time is near when some definite method of revising or of 

 disposing of the numerous local stratigraphic designations will be de- 

 manded by the advance of detailed correlations. 



Concerning overlapping formations. — Speaking of formations, one very 

 common difficulty is to decide to what extent an overlapping formation 

 may be reduced in thickness before another designation is desirable ? In 

 the case of a formation that is justly described as an indivisible strati- 

 graphic or lithologic unit — meaning that it does not include a measure- 

 able stratigraphic hiatus — all will probably agree that the same name is 

 properly applicable from the locality of maximum, development to its 

 vanishing edge. But when the formation is not strictly a unit and it is 

 found to comprise two or more lithologically distinguishable members, or 

 one or more stratigraphic hiatuses, or when the lithic characters of the 

 whole or of some part of the formation change laterally, then opinions 

 are likely to differ in deciding the nomenclature of extensions of beds 

 falling within the limits of the formation in question. If it is a simple 

 case of overlap of a formation consisting of two or more members, none or 

 only the lower of which has been given a subordinate name, the same name 

 should be used so long as any part of the lower member is recognizable 

 in the section. Beyond the point of its extinction a new name, providing 

 beds corresponding to the remaining member or members have not re- 

 ceived a formal designation elsewhere, is commonly advisable. Should 

 the formation be one of three members its reduction by overlap to the last 

 makes a third name desirable; but only when the middle and upper 

 members are united under the second name at the intermediate locality. 

 Should all of the three members, or both of the upper two, or only the last. 



