EEAPPEAEANCE OF GENERA. 189 



indirectly, what is there, in view of the persistence of the nautiloid 

 type, to preclude the possibility of the same genus being re-evolved ? 

 There may be a sufficiency of reasons with which we are not ac- 

 quainted ; but if, with our present limited knowledge, we are unable 

 to indicate what these may be, it is scarcely fair to insist, a priori, 

 upon their existence. It is true, we have no special reasons for as- 

 suming why the Nautilus should become modified into a Goniatites ; 

 but this of itself is no proof that it may not. Numerous instances 

 of genera widely separated by formations in which no representative 

 of their tribe is to be met with are known to every paleontologist, 

 but perhaps no more remarkable example is presented than that 

 of the genus Nummulites. It has been well said by Mr. Brady " 

 that "there are few time-marks in the geological record that have 

 been regarded as better established, or more definite, than the first 

 appearance of the Nummulite, at or near the commencement of the 

 Tertiary epoch," and so little, in fact, is known of any of the ante- 

 cedent forms, that many, if not most, paleontologists of the present 

 day still hold to the correctness of the view here stated. The re- 

 searches of Gumbel and Brady have, however, placed beyond all 

 doubt the existence of at least one Jurassic species (Nummulites Ju- 

 rassica, Franconia) and one Carboniferous (N. pristina, Belgium), 

 and not unlikely a somewhat doubtful form, N. variolaria, var. 

 prima, described from the Cretaceous rocks of Palestine, will, on 

 further investigation, prove to be a true member of the genus to 

 which it has been referred. That we have here an instance of 

 generic reappearance it is impossible to affirm, but it is certainly 

 almost inconceivable, whichever way it be considered, that a group 

 of animals, so extensively developed as are the Nummulites in the 

 Tertiary deposits, should have left practically no traces of their 

 existence behind them in the deposits next preceding the Tertiary, 

 the Cretaceous, when their ancestry dates so far back as the Car- 

 boniferous epoch. It is scarcely possible that at no period of time 

 between the Carboniferous and Tertiary epochs should the condi- 

 tions for their development have been favourable ; and equally im- 

 probable does it appear that, if such development actually did take 

 place, we should so thoroughly lose sight of their remains. Grant- 

 ing the absolutely unfavourable conditions, however, can it be 

 readily imagined that a few miserable forms, evolved 'at an entirely 

 unpropitious moment, should have battled through the struggle for 



