EXTINCTION OF AKIMAL GROUPS. 199 



or to a distinct class which yield their places to other species which 

 have been improved and modified, a few of the sufferers may often 

 long be preserved, from being fitted to some peculiar line of life, or 

 from inhabiting some distant or isolated station, where they have 

 escaped severe competition. For instance, some species of Trigonia, 

 a great genus of shells in the secondary formations, survive in the 

 Australian seas ; and a few members of the great and almost extinct 

 group of ganoid fishes still inhabit our fresh waters. Therefore the 

 utter extinction of a group is generally, as we have seen, a slower 

 process than its production." 



There are instances, however, in which the extinction of certain 

 animal groups is generally considered to have been very sudden. 

 The trilobites and ammonites among invertebrates, and the dino- 

 saurian reptiles among vertebrates, may be taken in illustration of 

 such cases. But even here a careful examination of the premises 

 shows that the suddenness of extinction is probably much more 

 apparent than real, and that, as the facts now stand, they by no 

 means sustain the inferences that have been drawn from them. The 

 trilobites, for example, are frequently stated to stop suddenly at 

 the close of the Paleozoic era, whereas, as a matter of fact, no 

 trace of trilobites has ever been found in deposits unequivocally 

 newer than the Carboniferous. A whole period (Permian) — true, 

 a comparatively insignificant one — therefore, still intervenes be- 

 tween the extinction of the order and the close of the Paleozoic 

 era. But, again, even ■with the Carboniferous period the extinc- 

 tion is far from being sudden. Of the numerous genera which 

 so eminently characterise the Silurian fauna, only two genera, 

 Phillipsia and Proetus, both of them restricted to a comparative- 

 ly insignificant number of species, survive the Devonian period.* 

 These, together with two other genera now for the first time in- 

 troduced, Grifiithides and Brachymetopus, constitute the entire 

 known Carboniferous trilobitic fauna ; and of this limited number 

 only one genus, Phillipsia, and that apparently in America alone, 

 passes up as high in the Carboniferous series as the Coal-Measures. 

 We thus see how very gradual, rather than abrupt, has been the final 

 — if final — extermination of this order of animals. 



Nor do we have that sudden downfall, either in or from the 



* Since the above was written, Professor Claypolc has announced the dis- 

 covery of Dalmania in the Waverly Group (Lower Carboniferous) of Ohio. 



