OF THE ELASMOBKANCHS. 



65 



regions ; second, we can hardly expect that the same species would return after the lapse 

 of so long a period. As a matter of fact, no species that existed during the Palaeozoic is 

 known from any post-paheozoic formation. 



In order to illustrate more clearly the chronological distribution of the Elasmo- 

 hranchs of the world, I have prepared the accompanying table, which shows the genera 

 that existed during each geological period. As in the case of the diagram, the ichthyo- 

 dorulites have been excluded ; likewise a few unimportant genera, mostly such as 

 existed during only a single period, and most of them belonging to the Subcarbonifer- 

 OUS. The genera are arranged under their family names, without reference to their 

 special relationships to one another. The name of every genus which occurs in more 

 than one period is repeated in the proper column. If it is not found in the period 

 following that of its origin, but is found in a still later period, its absence in the 

 intermediate period is indicated by printer's "leaders." All of the genera of the 

 Pliocene column still exist ; as do also those there indicated by leaders. Furthermore, 

 two or more genera whose names occur in the same horizontal line have no special rela- 

 tionship to one another ; that is, it is not here meant that they stand in the relationship 

 °f ancestor and descendant. 



In viewing this table we are, first of all perhaps, struck with the blankness of the 

 Triassic column, only four genera there appearing ; and these furnished the forty species 

 which existed in the Triassic waters of Europe. The Permian period is somewhat bet- 

 ter supplied with genera, although these included only about half as many species as 

 (1 'd the three genera of the Triassic. However, the teeth named Dittodus quite cer- 

 tainly belong to other genera of Pleuracanthidse. Hence, we see that not more than 

 trom ten to fifteen genera of Elasmobranchs are known to have existed during the Per- 

 lni; iii period, a meagre number in comparison with the numerous genera of the Sufocar- 

 boniferous. 



Again, examination of the table shows that, with the exception of Hybodus, no 

 genus which existed during the Palaeozoic era continued on into the post-paheozoic 

 times. The status of Hybodus as a Paheozoic genus is open to some doubt and will be 

 considered later. We see, therefore, that whether the Elasmobranchs which bad (lour- 

 19 hed to such an extent during the Subcarboniferous period had remained around the 

 Shores of the northern hemisphere during Permian times, but for some reason left only 

 scanty remains ; or whether they bad been driven to other regions of the globe and 

 returned during post-paheozoic times ; when we find them again, if it is their descend- 

 ants which we find, they have meantime become transformed both specifically and gener- 

 ically. 



